There was a time when we believed that the Atom was the smallest possible particle in the Universe.
“The idea that all matter is made up of tiny, indivisible particles, or atoms, is believed to have originated with the Greek philosopher Leucippus of Miletus and his student Democritus of Abdera in the 5th century B.C. (The word atom comes from the Greek word atomos, which means “indivisible.”) These thinkers held that, in addition to being too small to be seen, unchangeable, and indestructible, atoms were also completely solid, with no internal structure, and came in an infinite variety of shapes and sizes, which accounted for the different kinds of matter. Color, taste, and other intangible qualities were also thought to be composed of atoms.
While the idea of the atom was supported by some later Greek philosophers, it was fiercely attacked by others, including Aristotle, who argued against the existence of such particles. During the Middle Ages in Europe, Roman Catholic theologians were heavily influenced by Aristotle’s ideas, and so atomic philosophy was largely dismissed for centuries. However, the Greeks’ conception of the atom survived, both in Aristotle’s works (his arguments against) and in another classical work by the Roman author Lucretius, De rerum natura (“On the Nature of Things”), which was rediscovered in Europe at the start of the Renaissance.”
Now, this brings us to a wonderful video that gives us an idea of how small an atom really is: