Pym Particles
Pym Particles are a class of subatomic particles discovered in the 1960s by Hank Pym and Janet Van Dyne. They are capable of increasing or reducing size while retaining the total mass. The discovery won Pym the 1971 Nobel Prize for Physics.
Pym was able to isolate these size changing effects to create the Ant-Man Suit and Wasp Suit which allow the wearers to change their size. He also used them to create Pym Disks, which apply the same size changing effects as the suits to an object they touch.