Why is space a vacuum ?
In: Planetary Science
You are asking a question that comes down to “Why are things the way they are?” to which the answer is “If they weren’t the way they are, then you wouldn’t be here to ask.” There is no why to why things aren’t different. I can explain why the vacuum is still a vacuum but past that, you will merely need to accept that it is.
A vacuum is merely a space devoid of matter. Space mostly fits this description, being mostly empty. The universe however is not a vacuum. We know that that after the big bang, matter(and antimatter) was spread to all corners of the universe. That matter would come together through the force of gravity to form planets and stars and everything you see in the nights sky and more. Gravity keeps everything together and at least for the next few trillion years, matter will end out over the vacuum.
But the amount of matter is a lot less than the amount of space. While gases and dust are lost by planets into the vacuum the amount is a drop of water compared to the endless ocean of space. It will never fill it up. This is part of the idea behind the eventual end of the universe, The Big Freeze or heat death. Under heat death, the last stars will burn out, the last blackholes will fissile as they expel radiation. All matter will be too far apart from each other to interact, cold. Only then will the universe be a vacuum once more, with a thin layer of matter throughout.
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