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Indeed, the only hint they would have that they were not alone would be the "weak" gravitational effects of all other existing matter pulling at the fringes of their visible universe. Perhaps they'll refer to this phenomenon as "dark energy".

The galaxies in our local group are gravitationally bound.

Right, but isn't the expansion increasing? So if they don't merge into one big galaxy at some point the expansion would out run gravity?

> In cosmic scales of many billions of years, observers on every galaxy will eventually think that their galaxy is the only one that has ever existed.

Unless of course some of them manage to find ways to preserve information and keep records over those billions of years. The lost galaxies will be known from that.

But the information would still be forever locked away in its own galactic island. Even transmitting at the speed of light, the information could never reach any other galaxy. So no, in this far future, a galaxy would only be able to know itself.

We have records of other galaxies right now. Assuming we manage to preserve those and humanity over these kinds of time scales, our descendants would be able to access those records and learn that other galaxies once existed that are beyond their cosmic horizon now.