Headcanon: Pym/Yellowjacket Particles
Pym Particles are a rare group of subatomic particles with the ability to shrink (or, with a little modification, grow) any matter that they come into contact with. Their discovery was made by Dr. Hank Pym, during a point in his life where he longed to throw off the constraints of the mainstream scientific community and pursue his more creative ideas - it is even believed that the reason he chose to use himself for the first human trials was due to a strong desire to simply shrink completely away from modern society as a whole.
After having discovered them, Hank devised a means to locate and collect more, storing them in stasis containers designed to prevent them from activating until their needed time. Though many have tried, Hank refused to reveal the means of the Particles’ discovery or the finer workings of his equipment - Hydra and SHIELD’s top scientists and spies have not even come close to cracking the formula.
In his current mental state, Hank/Yellowjacket’s views towards the particles fluctuate - some days he believes he was the ‘true’ discoverer of the particles, other times he believes he merely stole it before ‘killing’ Dr. Pym. In both these mindsets, he refuses to use the name ‘Pym Particles’, insisting that they are ‘Yellowjacket Particles’, and has gone to the length of developing a yellow-hued dye for the stasis containers in order to further bring the point home of their new classification.