We observe that the matter density in the universe is decreasing with time. As far as we know, this looks the same from every point in the universe: galaxies mo

We observe that the matter density in the universe is decreasing with time. As far as we know, this looks the same from every point in the universe: galaxies move away from each other. The speed at which they move away from each other doesn’t depend on where in the universe you are. (It only depends on the time, as the speed is slowly increasing.) There is no center from which they move away from. Or alternatively, every point in the universe is such a center. It’s like when zooming into a grid. It doesn’t matter at which point of the grid you zoom in, the grid expands in the same way regardless. The grid has no center, and also no boundary. Due to the laws of general relativity, extrapolating that observation into the past, the implication is that the matter in the universe was ever denser the further you go into past. Furthermore, there is a point in time where it becomes infinitely dense. This is what we call the Big Bang. Because our equations break down at that point, we don’t know what happened at that point in time, or before, or whether there was a before. However, observational evidence like the cosmic microwave background strongly support the theory: https://en.wikipedia.org/wiki/Big_Bang#Observational_evidence https://en.wikipedia.org/wiki/Big_Bang#Observational_evidenc... Note that “infinitely dense” does not imply that the universe shrinks to a single point at that time. Rather, it was infinitely dense at every point. (Again, this is like zooming out from an infinite grid infinitely far. The grid always remains infinitely large.) This means that the Big Bang happened everywhere at once. It’s a point in time, not a location.