The question of whether everything is made of atoms is fundamental to understanding the physical universe. The simplest answer is that all ordinary matter—everything you can see, touch, or measure easily—is composed of atoms. These atomic structures form the basis of the world we experience daily. However, physics reveals that atoms are not the final answer; they are composite structures, and much of the universe is comprised of things that are not matter, such as energy and fundamental forces. The full scientific picture requires exploring these exceptions, including the constituents of the atom itself and the mysterious components of the cosmos.
The Building Blocks of Observable Matter
The atom serves as the defining unit of matter, specifically the smallest unit that retains the chemical properties of an element. Each of the over one hundred elements on the periodic table is distinguished by the number of protons contained within its atoms. This type of matter, often called baryonic matter, accounts for all visible objects in the cosmos, including distant stars and galaxies.
Atoms rarely exist in isolation but instead form connections through chemical bonds to create molecules. For instance, two hydrogen atoms and one oxygen atom combine to form a molecule of water. This resulting substance has properties entirely different from its atomic components.
These molecules then aggregate into the various states of matter, such as the tightly packed structure of a solid or the loosely associated particles of a gas. The organization of atoms and molecules gives substances their macroscopic characteristics, such as density, temperature, and hardness. Solids maintain a fixed volume and shape because their constituent atoms are held in place by strong bonds. Atoms in liquids and gases move with increasing freedom. For everyday reality, the statement that everything is made of atoms holds true, as all familiar materials are built from these fundamental chemical units.
The Structure Within the Atom
While the atom is the building block of elements, it is not an indivisible particle, which was the original meaning of the term. Each atom is a composite structure containing a dense central nucleus surrounded by a cloud of orbiting electrons. The nucleus is composed of protons, which carry a positive electric charge, and neutrons, which have no charge.
Electrons are considered elementary particles, meaning they have no known smaller components, and belong to a class of particles called leptons. In contrast, protons and neutrons are composite particles known as hadrons. These larger particles are each made up of three even smaller constituents known as quarks.
A proton consists of two up quarks and one down quark, while a neutron is formed from one up quark and two down quarks. These quarks are held together by the strong nuclear force, mediated by particles called gluons. Therefore, the physical matter that makes up atoms is ultimately constructed from a small set of elementary particles: quarks and leptons.
What Isn’t Matter (Energy and Forces)
The most significant exceptions to the rule that everything is made of atoms are energy and the fundamental forces of nature. Energy, in forms like heat or light, does not possess the atomic structure that defines matter. Light is composed of individual energy packets called photons, which are force-carrying particles rather than matter particles.
Photons are a type of boson that mediates the electromagnetic force, allowing atoms to interact. Other fundamental interactions, like the strong and weak nuclear forces, are also mediated by specific bosons. These include gluons and W and Z bosons. These force carriers are not constructed from quarks and leptons and do not form atoms, yet they are a tangible part of the universe.
Beyond these known forms of energy and force, the vast majority of the cosmos is not atomic matter. Astronomical observations suggest that ordinary atomic matter accounts for only about five percent of the total mass-energy content of the universe. The remaining 95 percent is attributed to two unseen entities: dark matter and dark energy.
Dark Matter and Dark Energy
Dark matter makes up roughly 27 percent of the universe. It is a form of matter that does not interact with light and is not composed of atoms. Scientists hypothesize that it is made of new, undiscovered types of particles that interact only through gravity and possibly the weak nuclear force. Dark energy accounts for approximately 68 percent of the universe. It is not matter but a mysterious form of energy believed to be driving the accelerated expansion of space.