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13.7 Billion years ago, all of existence fit into a region of space smaller than an atom. Then, space exploded is size and variety, and the immense processes of the cosmos was set in motion.
Kinda. The truth, as always, is much more complicated and interesting.
Let's take a closer look
.We should start backwards with how we know it's expanding. When we look at the stars, the reason we can't see all of the billions of galaxies surrounding us with the naked eye is partially due to the distance, but also because of what we call Doppler redshift. Reliant on the principle in General Relativity that states that light waves must always be traveling at the speed of light relative to any given observer in the universe (which brings up a lot of fun scenarios on its own, but that's a story for another day), the light leaving those galaxies must maintain that speed to us while being emitted from an object moving startlingly quickly in the opposite direction. This leads to us measuring the light as stretched out and the object itself looking far more deeply-red than it would be up close, often far past the range of light we can see into the infrared spectrum. Even more astounding, not only do the farther galaxies appear younger (as we'd expect from looking at them with light that took a really long time to get here), but they seem to be moving away from us more quickly than the closer ones. This implies that this expansion isn't centered at any one point in space, but is more likely to be an expansion of space itself. Whatever caused this must have been an incredible force.
The story continued logically from there. If everything is expanding, then, obviously, everything was much more tightly-packed in the past. This seems intuitive until you come across all of the relativistic problems with packing all of the energy of the known universe into a single point. It's not that we don't have any evidence for it- after all, there's a dim level of microwave radiation emanating from every point in the sky behind (and, therefore, older than) the youngest and most distant galaxies to support it. The issue comes when we try to look past it.
Going back to general relativity, space and time are two aspects of the "fabric" of the universe, which bends fairly easily. All you need is mass. This is how we get gravity, and at most levels, it manifests as objects pulling towards each other relative to their collective mass. If you put too much of it into one spot, though, and spacetime starts distorting in far more unpredictable ways. Black holes, for instance, are considered inescapable not because anything is pulling objects further into it, but because space starts to become time-like: unidirectional and irreversible. Our models even predict the inverse effect of time becoming multidimensional and transversible like space. Unfortunately, even our best struggle to predict what the early universe of essentially infinite mass in a singularity would create. It's entirely possible that time itself didn't exist before then. There may be no real way to know what existed before the universe, if anything did, and therefore no way to learn where it came from.
Regardless, we know that it did start, we know that it did start, it did so violently, and we can see how if we know not just where to look, but how to look. Thirteen billion years is a long time, but it's a road we know how to travel.
