By my extreme size, I escape your eye
The nature of every thing, is but many of I
Since my unveiling, the inexact came by
And there came a new discipline to unify
What am I?
So there are answers for the atom and radiation. Both are very close, but arguably jhabbott got closest with their guess of elementary particles.
So, my own answer:
Particles that are, elementary
Finer than a germ, so small you can't see
Apples ants and me, all comprise many
For the inexact, that's uncertainty
What to unify? Quantum gravity
By "escape your eye" I did mean that one can't see it with naked eyes.
The second sentence of course means that it is a building block of matter (everyday or not).
While two answers are right that the third sentence refers to Heisenberg's Uncertainty Principle or the probabilistic nature of quantum-scale stuff (one is the consequence of the other, right?) the answer that guessed the atom made a mistake here: that didn't exactly come by with the concept of the atom, rather this appeared with the rise of quantum mechanics.
Regarding the new discipline to unify, it refers to the discrepancies between quantum mechanics and general relativity that lead to many different theories trying to resolve the problem. It can mean to unify quantum mechanics with relativity, or to unify those new theories and come to a single, widely agreed physical model again.