Distances to galaxies
It is the expansion of the Univers resulting from the Big Bang, the explosion within which our Universe supposedly formed, which allows one to measure distances to galaxies. The more distant they are, the faster they move away from us.
The closest galaxies to the Milky Way are at millions of light-years in distance.
The more distant are galaxies, the smaller they appear on the sky, and the harder it is to make out their shape.