This is a very basic question; yet, I don't know much about comics and I want to know: how did Flash (Barry Allen) create a whole new universe?
Spoilers from DC Rebirth!
TL;DR: No; while he thinks so, there were much more powerful players on the board.
In DC Rebirth #1 we see Wally West (3rd Flash) floating stranded in Speed Force. He seems to be unable to break out, because he has no tether to the world - everyone has forgotten him after the Flashpoint; he also seems to possess some knowledge/insight/gut feeling about what really happened during Flashpoint.
Long story short, Wally is able to remind Barry Allen of himself, and after Barry pulls Wally out of Speed Force, he reveals that it was not, in fact, Barry who messed up everything during Flashpoint, but someone else:
What Wally tells also explains why some canonical couples are absent in New 52, e. g. Clark and Lois, Oliver and Dinah, Bruce and Diana (?), et cetera.
One may already have guessed the identity of the unknown being when we see Pandora calling someone a heartless monster (also implying that she was tricked) and being killed, and in some of the following panels:
The description "heartless monster" has been given before to Dr. Manhattan (though I can't find the reference now), and the way she is killed is Dr. Manhattan's trademark:
The smiling face pin is a recurring symbol throughout Watchmen (both the film and the graphic novel); the conversation on the final panel is copied verbatim from Alan Moore's Watchmen #12 (1987):
Some background on Watchmen and Dr. Manhattan:
Watchmen was published by DC in 1986-1987; there was an eponymous film in 2009, and a series of comics titled Before Watchmen in 2012-2013. Generally, film follows the source pretty closely (extended or ultimate version, not the theatrical cut - you'll spend 3 hours but it's worth it), so you'd be good with watching it (although it may not convey the "same" atmosphere), but the most important parts regarding Flashpoint are linked to Before Watchmen: Doctor Manhattan.
Watchmen is set in the Cold War era. There are no superpowers, only caped/masked vigilantes and villains. The only superpowered being there is Dr. Manhattan, created when Jon Osterman, a physicist, is locked in an "intrinsic field generator"; Jon Osterman is disintegrated, but some time later he reassembles himself (from subatomic particles) as Dr. Manhattan.
Doctor Manhattan is actually almost omnipotent - he can manipulate matter in any way he wants, and he can travel in time and perceive time as if he was in every moment of it at once.
In Before Watchmen: Doctor Manhattan he travels back in time to prevent nuclear doomsday (because it's a big deal there), basically erasing every probability where the rockets are launched (and thus "It all worked out in the end").
Dr. Manhattan's motivation and how exactly he caused Flashpoint and created The New 52 is yet unknown, although Dr. Manhattan does disclose his plans in the end of Watchmen (as shown above), and later he is seen putting his plans to action in Before Watchmen: Dr. Manhattan #4:
I'd advise reading the Watchmen and Before Watchmen: Doctor Manhattan, just in case, and because those are great comics (and the film too).
The answer is yes and no.
Originally Flashpoint and the proceeding New52 reboot were directly caused by Flash; however, because New52 was so horrible and as result, was received horribly, DC tried to fix what they did wrong with their first attempt with Convergence, which explained that there is a Multiverse of Multiverses, all of which are ending where in some fraction of various Earth were preserved at various points in their timeline and brought to this new world to fight for their survival which eventually lead to the creation of a new Earth with a New Timeline with characters from other Timelines (such as Superman and Lois Lane from the Bronze Age "pre-New52"). I believe there were 4, but I'm not 100% sure on what exactly happened. Anyways, these Convergence events did pretty much nothing, but we're supposed to assume that it altered the timeline somehow, due to the fact that messing with Time always has consequences.
Anyways, at the end of Convergence nothing changed, and most of the out-of-time characters were sent back to their places of origin or something like that.
While the event and the end result didn't really do anything, the fallout of Convergence set the stage for Rebirth, which can be looked at as sort of a phase 2 of DC's return to the bronze Age, where we are told that the world they are in now is still the world of the Bronze Age comics, but the history has been altered by the Watchmen in some way.
So between 2011 and early 2015 the answer to your question is, Yes, Flash is directly responsible for the New52 coming into existence; however from mid-2015 and onward the answer is No, Flash was used as a pawn by the Watchmen for some reason while they altered the DC multiverse's timeline/de-aged/erased their memory.
Yes, in the original writeup which lead to the existence of the New 52 (also known as the DCnU or Nu52) Universe, Barry Allen's story Flashpoint is the event which precipitates the reformation of what was at that point called the Post-Crisis Universe into the rewritten, everyone-is-wearing-armored-costumes version of the New 52 which started in 2011.
"Flashpoint" is a 2011 comic book crossover story arc published by DC Comics. Consisting of an eponymous core limited series and a number of tie-in titles, the storyline premiered in May 2011. The core miniseries was written by Geoff Johns and pencilled by Andy Kubert. In its end, the series radically changes the status quo for the DC Universe leading into the publisher's 2011 relaunch, the New 52.
This rewrite, removed dozens of favored characters and replaced them with new versions of them. Two major changes included Barbara Gordon returning to her role as Batgirl and thus the dissolution of her identity as Oracle. The disappearance of Cassandra Cain, the former Batgirl, the return of Barry Allen and the loss of Wally West as the primary Flash in the DC Universe.
It also established during its tenure an event called Convergence which revealed a larger Multiverse of multiple worlds where what were once worlds before the Crisis, had been re-established as unique worlds again.
Since the mashup New Earth created by the multiple Crisis (Crisis on Infinite Earths, Infinite Crisis and Final Crisis failed to deliver the goods of putting all of the Multiverses worlds onto one planet, DC intelligently used Convergence to retcon away all of the problems with the failed Crisis events and went back to the Multiple Worlds theory of the DC Universe.
The Pre-Flashpoint Universe was considered somewhat of a failure because no one was able to write a cohesive history capable of merging all of the stories of the multiple Earths into one and keeping it coherent. Some aspects worked but others (like the Legion of Superheroes) failed to launch successfully.
- In the New 52, Prime Earth returned to being Earth 1, and other Earths also began telling their own stories such as the Earth 2 saga where Darkseid kills the DC Trinity of Batman, Superman and Wonder Woman and the heroes of that world are forced to step into new roles to replace them. Earth 3 once again returned to being the home of the Crime Syndicate, with only a few changes depending on writer requests for characters.
In 2011, during the Justice League, Darkseid War, the nature of the DC Universe was revealed in print, something rarely done. Because the character is Metron, a being whose origins are hinted at being from outside of the DC Multiverse, we are given a rare view of the DC Universe from the outside perspective.
This first panel reveals the struggles of the Pre-Crisis Universe against the threat of the Anti-Monitor whose goal was to crush the DC Multiverse back into a single universe (which he believed was the natural order of things, disrupted by the renegade Malthusian Krona.) All of the heroes of Earth-One, Earth-Two and all of the other heroes who could be gathered and weren't killed struggled to prevent the collapse of the DC Multiverse. They were unsuccessful.
The new collapsed universe became what is known as the Post-Crisis Universe which tried to place all of the heroes of the DC Universe on a single world, dubbed Prime Earth. DC would try two more times with later Crisis events to reconcile the DC Universe timelines with Infinite Crisis and later Final Crisis.
- The last panel on the right is the Flashpoint aspect of the DC Universe as the failure of Final Crisis to resolve the issues of the DC Universe would result in the creation of the New 52 Universe (Seen below).
Overall, the Flashpoint-resulting New 52 was considered a failure in that it did not create enthusiasm for the DC Universe. It was filled many (what people felt were unnecessary) crossovers and many readers found it boring and poorly representative of minority characters.
Crisis on Infinite Earth Entry on Wikipedia elucidates:
At the 2008 New York Comic Con, Dan DiDio said that Crisis on Infinite Earths was the first of a trilogy of "Crisis" limited series describing stages in the development of the DC multiverse; Crisis on Infinite Earths represented the "death of the multiverse", Infinite Crisis the "rebuilding of the multiverse" and Final Crisis the "final saga of the multiverse".
In an October 2, 2011 post on his Facebook page, DiDio implied that Crisis On Infinite Earths, Infinite Crisis and Final Crisis did not occur in the DC New 52 universe. According to John Lichman's "DC New 52 Timeline So Far", this clarified an earlier assertion by DiDio: "Brace yourself, but after further review, there have been no Crisis events in the New DCU".
The Convergence (2015) crossover officially retconned the events of Crisis after heroes in that series went back in time to prevent the collapse of the Multiverse. However, the actual events that occurred when the Crisis' erasure of the multiverse was averted have yet to be disclosed. Whether this means that the pre-Crisis Supergirl and Flash did not die during the recalibration has yet to be clarified. Also left unanswered is the relationship between the Pre-Crisis, Hypertime, 52, and post-Flashpoint multiverses.
Ultimately in late 2015 it was decided the DC Universe would be rebooted into what is now called the DC Rebirth. Rebirth has tried to go back to heroes which are both Human, approachable and able to be related to by a wider and more diverse audience.
Not much has been written about the Multiverse at large while we are learning what brought about the Rebirth. So far, hints appear to imply the Rebirth was in some fashion caused by or affected by the godlike entity Doctor Manhattan from the previously segmented Watchman universe.
Since the Watchmen are being folded into the DC Universe, writers decided to use this opportunity to expand the DC Universe (likely keeping it a Multiverse, and including the rest of the Watchmen into the DC Universe.) Rumors also say the Authority and other Wildstorm products may splinter off onto their own worlds as well.
If you want, you can read the synopsis on Wikipedia.
The page also states
In its end, the series radically changes the status quo for the DC Universe leading into the publisher's 2011 relaunch, The New 52". You see that Barry Allen was indirectly responsible for New 52 Reboot.