The Expanding Universe
Edwing Hubble and his takes on the universe
In the 1920s, when astronomers began to look at the spectra of stars in other galaxies, they found something most peculiar: there were the same characteristic sets of missing colors as for stars in our own galaxy, but they were all shifted by the same relative amount toward the red end of the spectrum.
To understand this, read the Doppler effect.
There are 3 different kinds of models that obey Friedmann's two fundamental assumption.
Present rate of expansion and present average density
All of Friedmann solutions have the feature that at some time in the past, the distance between neighbouring galaxies must have been zero. At that time (big bang), the density of the universe and the curvature of space-time would have been infinite. Because math cannot really handle infinite numbers, this means that the general theory of relativity predicts that there is a point in the universe where the theory itself breaks down. Such a point, in math, is called a singularity.
Even if there were events before the Big Bang, one could not use them to determine what would happen afterward, because predictability would break down at the Big Bang.