In current physics, matter is fundamental, so it should break down into particles, and it seemed to. Electrons and quarks are examples, and even photons that have no mass or charge are said to be particles. This method, of breaking apart matter to find out what it is made of, began with the atom.
Initially, atoms were thought to be indivisible, like little billiard balls, but when Lord Rutherford fired alpha particles at a piece of gold foil, they mostly went straight through and only a few bounced back. It turned out that 99.9999…% of the mass of the atom is in its nucleus, and the rest is just a cloud of tiny electrons whizzing about.
Bohr then suggested that the atom is like a solar system, but held together by electrical forces not gravity. Yet electrons routinely pass right through each other which planets don’t, and different electrons can occupy the same orbit which planets can’t, and planet orbits are elliptical but electron orbits are perfectly spherical, so the atom isn’t like a tiny solar system! The modern view, called the standard model, summarizes what physicists know about atoms.