What is the main gas in a helium balloon that makes it float?

Helium, which floats because it is about seven times less dense than air - the balloon is not being pushed up so much as the heavier air around it is falling past and displacing it. It is the second most abundant element in the universe and genuinely scarce on Earth, because it is light enough to escape the atmosphere entirely and never comes back.

Effectively all of it is dug up. Helium on Earth is produced by the radioactive decay of uranium and thorium deep underground and accumulates in gas fields over hundreds of millions of years, so it is extracted as a by-product of natural gas from a handful of places - Qatar, Algeria, Russia and above all the United States, which built a strategic reserve in Texas for airships and then spent decades selling it off cheaply by act of Congress.

That is why party balloons are a slightly uncomfortable use of it. Liquid helium at four degrees above absolute zero is the only practical coolant for the superconducting magnets in MRI scanners, and there is no substitute; it is also needed for semiconductor manufacture and for cooling detectors. Supply shortages have repeatedly disrupted hospitals, and the gas released from a balloon is gone from the planet.