More accurately, that everything comes from and is shunyata (emptiness). Emptiness is also not nothingness, which is a different thing. Emptiness (of svabhava: "self existence") means infinite potentiality, since it is what makes anything possible. Nagarjuna proved that if something is not empty of self existence then it would annihilate itself and be unable to have any impact on our reality. The only reason that things can (seem to) exist is because nothing exists in itself, so it is impossible to distinguish between say, an apple, and the entire universe.

I think it’s just imagining how big the universe is…and then imagining that, but multiplied with space in between. The possible collapse of it all is a fun bonus.

A kind of megalophobia? Mixed with what cosmic horror invokes, the feeling that you are nothing in an indifferent universe. Knowing just enough about the universe to know you are nothing.

Universe is not exactly indifferent. Neither indifferent nor not-indifferent. David Deutsch offered some consolation to me, because when you take in account multiple dimensions there is also a vast quantity of Earths out there "somewhere".

The idea that there are multiple parallel earths is fun to think about, but know that it is an unfalsifiable hypothesis and arguable is outside the realm of science (and close to the realm of meta-physics).

Is this knowledge particularly useful in some way in this case? Does it prompt any particular action in response?

If you could figure out the distribution of universes, somehow, and figure out the distribution intelligences across them to a high enough fidelity to predict their actions (for example, by crunching numbers with a star-sized-computer, predicting what modal source code of some universe-spanning AI in a parallel universe looks like, and running it), you could in principle bargain in a cross-universe manner that leads to both parties/universes gaining value. Say you highly value diamond, and want to turn all matter in your lightcone to diamond; you've got 99% of it, but the last 1% is extremely expensive. Your counterparty values happy sentient life, and has dismantled 99% of the stars in their light-cone to make very long-lasting artificial habitable worlds, but the last 1% is the hardest to use- but would be easier to turn into diamonds than your last 1% of usable matter, and your last 1% of usable matter is easier to turn into living things than diamond. If you could somehow both predict with high confidence that a universe containing your counterparty exists, you could both gain value by using that last 1% to help the other, in a prisoner's dilemna sort-of-way.

(One possible avenue against defection is that if I can simulate you well enough, I know by definition whether you'll defect or not.)

This whole process is called 'timeless acausal trade', and it's pretty interesting.

It's the damn truth (the only true "true") at the bottom of everything. But the anxiety goes away after a while.