Beyond the Trojan Reach
It was a period where worlds were cut off from one another, technology was lost and the population on many worlds simply failed to survive. Even though the Long Night “ended” over one thousand years ago, the galaxy is only just emerging from this period.
The concept of the Long Night is more than a little centered on the Imperium. Other major races (Aslan, K’kree, etc.) were not affected by the decline of the Second Imperium, nor were other branches of Humaniti which were not part of the Imperium.