The Beta version now has nebulas. They will only appear in a new game, old games will reload the same as they were before the update.
In a couple days, as long as no issues are found, I will update the default version of Star Explorers to have nebulas.
Nebulas are large gas clouds that appear in outer space. They currently don't actually do anything, they simply add a little variety to the appearance of the game. They are formed procedurally along with the rest of the galaxy at the beginning of a game, and will persist when the game is saved and loaded again.
You will notice also that the space background has been slightly updated as well. Previously there were some bright spots that looked a bit like stars. These have been removed.