I heard it in the dialogue in Star Wars movies. It goes something like "you do not understand the power of the dark side" or "do not underestimate the power of the dark side". What is so powerful about the dark side that they are referring to?
As dark side users (Sith) they obviously think the dark side is stronger than the light. If they didn't think that, they'd be Jedi.
There are a number of reasons why the Sith would think that the dark side is stronger. Here are just a few:
- Just a few Sith Lords (no more than 2-3 at a time) took over the Republic and nearly exterminated the Jedi.
- The Sith Master (Sidious) defeated the Jedi Grand Master (Yoda).
- The Sith Master quickly defeated three Jedi Masters who were trying to arrest him in a 4-on-1 lightsaber duel, and seems to have thrown the fight against the fourth only to tempt another Jedi (Anakin) to the dark side (and it worked).
- The dark side gives them abilities that the Jedi consider "unnatural", such as Force Lightning and the ability to resurrect loved ones (the latter is an ability Sidious claims that his master, Darth Plagueis, possessed).
In Legends, the Sith demonstrate even greater feats. A few of them, taken from this question, include:
The answer to this depends on your point of view, and also the context in which the words are spoken. The two best examples in the movies are spoken by Vader to Luke, and depending on the scenario, the phrase takes on a different connotation.
In The Empire Strikes Back, on the gantry in Bespin, it's obvious that Vader is trying to tempt Luke into joining him with the idea of how powerful he can be were he to turn to the Dark Side. This is in response to Luke rebuffing Vader's offer to train him and destroy the Emperor together.
In Return of the Jedi, on the landing platform on Endor, the conversation is ultimately a counterpoint to the above scenario. Luke has just asked Vader to come with him, to whit Vader replies
You don't know the power of the dark side. I must obey my master.
This is a completely different context for the word power - Vader is revealing how, via the Dark Side, he is powerless to resist the commands of the Emperor.
Both phrases are correct in the context in which they are used, but shows how the Dark Side is both the gateway to power as well as enslavement. As Yoda says to Luke on Dagobah,
If once you start down the dark path, forever will it dominate your destiny, consume you it will, as it did Obi-Wan's apprentice.
To use the Dark Side you have to use stronger emotions than the Jedi do such as Passion (if you count the as an emotion that's up to you), Anger, Hatred, and Pain (again that one is up in the air as an emotion but the Sith us it to channel the dark side so I'll keep it). In real life all of these things can cause spikes in adrenaline wich can cause sudden bursts of strength or speed. This is why the Dark Side seems stronger, it is. I mean we consistently see Sith being much more powerful than Jedi with it often taking 2-3 Jedi to take out 1 Sith.
Examples of this include.
- Sidious vs Mace Windoo and the council (Corucant)
- Maul vs Qui-Gon and Obi-Wan (Naboo)
- Dukoo vs Obi-Wan and Anakin (Grieves's Flagship)
Now as I've said in the past there are exceptions to every rule but most of the time fully trained Sith Lords cannot be beat by single Jedi
Examples of this include.
- Sidious vs Yoda (Corucant)
- Maul vs Qui-Gon (Tatooine battle not Naboo)
- Dukoo vs Anakin (Geonosis)
- Luke vs Vader (Death Star)
- Yoda vs Dukoo (Geonosis)
- Rey vs Kylo Ren (First Order Planet)
So in summation over all the dark side is stronger and the only reason the Sith don't win immediately is the sheer number of Jedi.
There is not real light or dark side of the force. There is only The Force. It is what you do that places you on the dark or light side.
The concepts that the dark side has more power is also not a real thing. It is quite simply, the Jedi practice humility and restraint and refuse to practice their skills in ways that could or would harm others or thenselves. Sith have no such compunctions about practicing less humble aspects of the force and use the force to affect actual harm, they therefore could be said to have more practice being bad.