Could be "Stealing Light" by Gary Gibson.
It's the first book in "The Shoal Sequence" series. Here's what the blurb says:
In the 25th century, only the Shoal possess the secret of
faster-than-light travel (FTL), giving them absolute control over all
trade and exploration throughout the galaxy.
Mankind has operated within their influence for two centuries,
establishing a dozen human colony worlds scattered along Shoal trade
Dakota Merrick, while serving as a military pilot, has witnessed
atrocities for which this alien race is responsible. Now piloting a
civilian cargo ship, she is currently ferrying an exploration team
to a star system containing a derelict starship. From its wreckage,
her passengers hope to salvage a functioning FTL drive of mysteriously
But the Shoal are not yet ready to relinquish their monopoly over a
technology they acquired through ancient genocide.
From a review, the part where the humans spot a disturbance in space:
This starts off early, with the revelation that humans had recently
detected a massive series of supernovae in the Large Magellanic
Cloud. In addition to creating a sense of foreboding and mystery, it
drives home the inhumanly vast scope of events, in both time and
space - it took 160,000 years for the light from this catastrophe to
reach human territory...
An extract from the book describing it:
The first supernova had appeared six years before, early in the autumn, and just a couple of days after Dakota’s sixteenth birthday.
It had blossomed like cold fire, briefly one of the brightest elements
in the night sky, before gradually fading out over the following
weeks. Then, over the next several years, dozens more had appeared at
irregular intervals, shining brightly for a few brief weeks before
again fading back into stellar anonymity. And all this had occurred
within a relatively tiny sector of a neighbouring galaxy.
~ Cover image from SfReviews.net