It's just mind boggling that all of the matter and energy that ever existed was there at the big bang. Which means that every degree of heat ever pumped out by the sun was there, but also every degree of heat from every star in our galaxy and every galaxy in the universe. It was all in one tiny spot. And every ounce of rock from here to the farthest reaches of the universe existed - it might not have been in rock form, but the mass was there. All in one tiny point.
The mind boggling part of it to me isn't just that you could have all of that energy and matter in one tiny space - or even the massive inflation that occurred in the fractions of a second in which the universe blinked into existence. That's hard enough to wrap your mind around.
But what really gets my head twisted is that the universe has a specific amount of mass and energy that isn't infinite, and doesn't change. Why does the universe have a mass of 1.468 gazillion tons, and not, say, 3.14? Or 1? The concept of "laws" in the universe has never made sense to me - a formula like e=mc^2 just seems absurd to me in the sense that there's no real reason for c to be 186,000 miles per second except that that's what it is. If there are arbitrary things in the universe, then how did they get arbited?
And are they universally consistent? If there are other universes, do they all have the same amount of information, or are there big universes and little universes? Is the speed of light the same from universe to universe? And while I understand that information is always preserved, if there is a big crunch, will the "next" iteration of the universe have the same amount of information?
Also blowing my mind is that the idea of "before space time" doesn't exist, but that there is a "beginning" to space time. That we can accurately say what happened in the first second of the universe, because that was literally the first second that ever existed, and nothing predated it.
The closest I can come to making sense to all this is to look at the universe as a balloon. It's easy to imagine the inflation of the universe like the inflation of a balloon, and it's pretty common to use that analogy to describe how things are expanding away from each other. It follows logically that the big bang is like the first bit of breath into the balloon, which took it from an infinitesimal speck to what it is today. But that's not right, because that scenario imagines a balloon growing into space that was already there, and occupying more and more space as time goes on. Which implies that there's something outside of that space for it to grow into. And that's not right.
Instead, it helps me to think of the universe as an already inflated balloon which is constant in size and shape. It doesn't change at all. If you take a cross section of that balloon at its widest point, you'll get a circle that's a certain size. Take a cross section at a different point, and you get a different, smaller size. At either end of the balloon, you have a balloon knot and a nipple thingy. A cross section at the very end of the balloon knot would be almost infinitely small. As you move closer to the center, it gets bigger and bigger, very rapidly at first and then more slowly until you get to its maximum size, and then the exact opposite.
Instead of thinking of those cross sections as happening from left to right, or from top to bottom, or however you're imagining that balloon, think of them as happening from beginning to end. Because time is just one dimension. From any point on the balloon, you could make inferences about the size and curvature and dimensions of the balloon, but sitting on the outside of the balloon, you could only truly see the stuff right around you, and it would all be curving away.
But if the universe has a finite beginning and a finite end - even if it's too large for us to see or even comprehend, so we can only see things one cross section at a time - then why does it have that beginning and end? Why is the universe set to last 28 billion years (or however long it is) instead of 42 billion or 1 trillion or some other arbitrary amount of time? Every finite aspect of the universe is arbitrary. How did it become what it is and not something else? I just can't get my arms around it.