No one knows for sure when it happened. There are a lot of theories and I have my own, but one day the earth stopped aggregating new material. Instead of the average 50,000 tons of material falling on the earth, the earth started losing mass instead of growing. No more space dust, meteors, meteorites, flotsam and jetsam of the universe. We’d entered a dead space, where space was empty, a void.
No one noticed, at first. No one is responsible for measuring that sort of thing anyway. What was noticed seemed funny. After all, we didn’t pay any attention to it unless we were decorating for a party. Helium. Free helium is light. Too light. Over time it escapes the atmosphere, and in the void, the earth began to shed its helium until there wasn’t enough left to fill a good sized balloon.
With the overall mass a bit smaller, hydrogen was next to make a great escape. Not a problem there, since there’s so much of it. The bad part being that hydrogen is an essential component of water. Free hydrogen we could live without; water, not so much. Other lighter elements also began drifting out into the void, and our atmosphere thinned. Living at sea level is now like living a mile high. And no one can live at a mile high any more, so we huddle at the lower elevation, in a tight band between the eroding coastline and anything above maybe 500 feet.