HELIUM
goodbye party balloons
Also goodbye MRI machines and particle physics since liquid helium is the most commonly used coolant for superconductive devices.
Removing all the helium from the universe might royally forget up our sun too, since 26% of its mass is helium. I don't know whether that would actually affect its luminosity (since helium doesn't fuse into anything else in a sun like ours), but it would probably forget something up.
