How do we know if distant Galaxies are still there

[Gamma986]

if it took billions of years for the light to reach earth?

[gregorx2001]

We do not know that they are still there. When astronomers look at galaxies billions of light years away they are looking billions of years back in time. To put it another way this ancient light really left that galaxy billions of years ago.

[Sam]

To my knowledge, the question of if gravity travels at the speed of light or instantaneously is still being resolved.

If gravity travels instantly, we can infer that the galaxies are still there because their gravitational effects are still present.

If gravity travels at the speed of light, we will not be able to tell because we're still getting the 'old' gravity, much like we are getting the 'old' light.