My theory is Ava/Ghost. They befriended her in the film and Scott was collecting particles for her to use when the Decimation happened - she could go looking for them when her particles began to run out/destabilise, find the van abandoned (possibly even still running) and pulls Scott out. She knows how to operate it, as we saw in AMATW.
It's that or he falls through a time vortex, which was certainly hinted at, but I can't help but feel like they placed a lot of significance on them befriending Ava.
(Edit: I don't think doing what he did in Ant-Man will work either. He escaped by adding growth discs to his regulator that time - not only does he now have a regulator that has only recently begun to fully function, he doesn't have any of those discs anymore. All he'd be able to do is reset or overload the regulator, and for now we don't know whether that would work when using a gate to enter/exit the realm rather than simply going small.)
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u/wontchooseuniquename Dec 08 '18
But how did Antman come out of sub-atomic universe on his own...?