We have several methods of calculating the distance of other galaxies from the earth (see Cepeid variables and type 1a supernova). From Hubble’s observations we know that the ratio of velocity to distance is constant and in this calculation we assume that the universe has been expanding since the big bang with all the constituents moving at constant velocities relative to one another. (That is the big assumption we are making we are assuming the velocity is constant and so the rate of expansion has been constant)
From the ratio of velocity to distance from Earth we measure Hubble's Constant.
Using the value calculated we get from now a large expanding universe with galaxies moving apart at high speed.
To the very distant past when , at the start of everything, the Big Bang when all matter was concentrated at a tiny point.
Other pages of notes and video on astronomy which may be useful are:
Units of distance notes and video Measuring distance by parallax/triangulation notes and video Life cycle of stars Geostationary and polar satellites notes and video Big Bang theory and evidence Development of the Universe after the Big Bang Real and apparent magnitude Hubble's Law and measuring distance notes and video Using Hertzsprung Russell diagrams notes and video Cepheid variable stars Type 1A supernova