Astronomers observe other galaxies in the Universe. Most of the material in the Universe is in the stars, gas clouds, black holes, planets etc in the galaxies.
In the 1920s, astronomer Edwin Hubble began to measure the speeds and directions galaxies move in. To everyone’s amazement Hubble discovered most galaxies are moving apart. The only way to explain this was that the Universe as a whole is expanding and carrying the galaxies with it. The galaxies are like currants in cake dough, as the cake bakes in expands and the currants move apart.
Hubble’s discovery led to the idea that the Universe exploded into existence, the Big Bang.