For most of human history, people believed the smallest thing in the universe was the atom. The word atom even comes from the Greek word meaning indivisible. But over the last hundred years, scientists have peeled back layer after layer of matter and discovered that atoms are not indivisible at all. They are made of even smaller pieces, and those pieces are made of smaller pieces still. The deeper physicists look, the stranger and more amazing the universe becomes. Today, the leading candidates for the smallest known things in existence are particles so tiny that they have no measurable size at all.

Atoms are made of three main types of particles — protons, neutrons, and electrons. Electrons orbit around a central nucleus made of protons and neutrons. For a long time, scientists believed these were the fundamental building blocks of matter. Then, in the 1960s, physicists realized that protons and neutrons themselves are made of even smaller particles called quarks. Each proton and each neutron contains three quarks, held together by an invisible force called the strong nuclear force. Quarks are considered fundamental, meaning we have no evidence that they are made of anything smaller.

A quark is so small that its size cannot even be measured with current technology. Experiments suggest that a quark is at least ten thousand times smaller than the nucleus of an atom, but it may have no measurable size at all. Scientists call particles like this point particles, meaning they behave as if they exist at a single point in space. Other point particles include electrons, photons (particles of light), and neutrinos. These tiny particles are not just small in size — they obey the strange rules of quantum mechanics, which means they behave in ways that defy ordinary common sense.

Modern physics has theories that suggest there might be even smaller things than quarks and electrons. String theory, for example, proposes that all known particles are actually tiny vibrating strings, far smaller than a quark. Other theories suggest that space itself may have a smallest unit, sometimes called a Planck length. So far, none of these ideas have been proven, because the technology to test them does not yet exist. The Large Hadron Collider, the world's most powerful particle accelerator, is one of the tools scientists use to look for evidence of smaller particles, but its discoveries are still limited by the laws of physics themselves.

Understanding what the smallest things in the universe are not only satisfies our curiosity but also helps explain how the universe works at its deepest level. Everything we see — from stars and galaxies to your own body — is built out of these tiny particles. The discovery of quarks transformed our understanding of matter and led to entirely new fields of physics. Knowing about quarks, electrons, and other fundamental particles reminds us that the universe is full of mysteries that we are still uncovering, one tiny piece at a time.