TL;DR: we saw a burst of energy (scientific name: luminous fast blue optical transient) that we previously thought only came from a star going supernova, but this burst came from the middle of nowhere in the sky and therefore was not caused by a star exploding. So now we know that these explosions (LFBOTs) aren't just made by stars.
Until a better theory comes along, I will assume we just witnessed our first warpcore breach. A moment of silence for the ship, and may her crew have gotten to the escape pods.