So I’m aware it’s because of pressure difference, but why does that matter? Why does the pressure try to equal out immediately and not just slowly wait as things move into it like they would in ambient pressure?
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At ambient pressure, air molecules are flying around at high speed, constantly bouncing against the walls and into each other. If you have a cloud of gas, its molecules will try to fly outward, but are soon bounced back into the cloud by the air molecules around it, and only slowly this random back-and-forth will lead to the cloud dissipating into the air around it.
If the cloud neighbors a vacuum though, there is no resistance at all in the direction of the vacuum. Any molecule that flies into the void will just keep flying until it hits the opposite wall, so the vacuum will fill up quite fast.
As an analogy, imagine you have a pawn on a chessboard, and you repeatedly throw a coin. At heads, you move it one row forward, at tails, one row backward. That’s the random motion of a gas cloud at ambient pressure. You will eventually reach the other side, but it will take a while. Now you do the same, but tails don’t count. That’s the expansion into a vacuum; you will reach the other side way faster.
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