Is there anything in between the galaxies?
Apart from intergalactic space...
Some stars may have once belonged to galaxies, but those which no longer do are called "stellar outcasts" or "intergalactic stars".
There is also this hypothetical unseen dark matter which makes up around 27% of the universe. Scientists have not figured out what exactly it does, but were able to tell there was unseen matter based on the motion of stars.