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According to the Law of Conservation of Mass, matter (and energy) can neither be created nor destroyed. So to answer your question about where all this matter came from, the answer, according to the Big Bang theory, is that it has always been here. Every molecule, every atom, every material object is composed of mostly empty space. If you sucked out all of the empty space in, let's say the Empire State building and left only the solid matter, it would amount to less than the size of a grain of rice, although it would be a super heavy grain of rice. So according to the theory, the whole universe, devoid of empty space, was once the size of an acorn. It then exploded and has been expanding ever since. A bigger question might be where did the empty space come from? According to the theory, space and time didn't exist until a fraction of a second after the explosion. The biggest mystery is figuring out precisely under what conditions they were created.
But just as a sidenote, one question I've always had is: if the universe is expanding, what is it expanding in? To say something is expanding is to suggest that there is a larger body outside of that thing. For example, I can fill a balloon with air and it expands. It only expands because it is in a much larger room, within which it has space to expand. But the orthodox concept of the universe is that it IS EVERYTHING, so there could be nothing outside of it for it to expand in.

[NOBLE]

According to the Law of Conservation of Mass, matter (and energy) can neither be created nor destroyed. So to answer your question about where all this matter came from, the answer, according to the Big Bang theory, is that it has always been here. Every molecule, every atom, every material object is composed of mostly empty space. If you sucked out all of the empty space in, let's say the Empire State building and left only the solid matter, it would amount to less than the size of a grain of rice, although it would be a super heavy grain of rice. So according to the theory, the whole universe, devoid of empty space, was once the size of an acorn. It then exploded and has been expanding ever since. A bigger question might be where did the empty space come from? According to the theory, space and time didn't exist until a fraction of a second after the explosion. The biggest mystery is figuring out precisely under what conditions they were created.
But just as a sidenote, one question I've always had is: if the universe is expanding, what is it expanding in? To say something is expanding is to suggest that there is a larger body outside of that thing. For example, I can fill a balloon with air and it expands. It only expands because it is in a much larger room, within which it has space to expand. But the orthodox concept of the universe is that it IS EVERYTHING, so there could be nothing outside of it for it to expand in.