Galactocentrism was established in 1925, which realised that our solar system is not near the center of the Milky Way. So, we are moving relative to the center of our galaxy.
In 1929, evidence was found that everything else is moving away from us. So we are moving relative to everything else.
In 1931, the Big Bang theory started superceding Galactocentrism, which was an acentrist model of the universe (where there is no center).
But moving relative to what?
Everything else.
Galactocentrism was established in 1925, which realised that our solar system is not near the center of the Milky Way. So, we are moving relative to the center of our galaxy.
In 1929, evidence was found that everything else is moving away from us. So we are moving relative to everything else.
In 1931, the Big Bang theory started superceding Galactocentrism, which was an acentrist model of the universe (where there is no center).
But what frame of reference?