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Q&A: Cosmology

I believe I understand the basics of the Big Bang Theory; as much as any layperson does. I can appreciate that we measure time from that point, though it escapes me why there isn't a "T minus 5, and counting...."
My question though is: though all matter was formed out of the Big Bang, with its gasses and gravities etc., where did the endless void come from?

Let us answer the question implicit in your first paragraph. The reason there is no "T minus 5 and counting" is because both space and time were created in the instant of the Big Bang. So it does not make sense to talk about the time before the Big Bang. Please remember however, that at the very earliest times, we really have very little idea of what happened. The laws of physics as we know them break down at a point of infinite density (known as a "singularity"). We do have compelling experimental data that shows us that the very early Universe was in a an extremely hot, dense state. That takes us back to a tiny fraction of a second after the Big Bang. But we don't know what happened earlier than that.

The definition of void is "containing nothing" or "not occupied." It is an interesting philosophical discussion to think of where a void comes from, but since by definition it is nothing, physics does not address this question. Physics only deals with ideas and objects that can be experimentally tested. If there is no matter, no energy, no space and no time (this is a void in astrophysical terms), then there is absolutely nothing for science to do.

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