The question of what constitutes all things moves beyond simple observation into the intricate theories of modern physics and chemistry. Science provides a hierarchical model that descends from the visible world to the fundamental components of matter. Our current understanding reveals that the objects we see represent only the initial layer of a much deeper structure. The nature of matter requires examining ever-smaller scales.
The Atomic Foundation
All familiar matter is built from atoms, the smallest unit that retains the properties of a chemical element. Every atom consists of a compact, positively charged nucleus surrounded by a diffuse cloud of negatively charged particles. Although the nucleus is tiny, it contains over 99.9% of the atom’s total mass.
The outer particle cloud governs how atoms bond and interact, which dictates the field of chemistry. The number and arrangement of these outer particles determine an element’s reactivity and state (gas, liquid, or solid). Different elements, such as oxygen and carbon, are simply different types of atoms defined by their unique internal structure. When these atoms link together, they create the diversity of substances in the observable universe.
The Subatomic Core
A closer look reveals that the atom itself is a composite structure, held together by three primary subatomic particles. The nucleus houses the positively charged proton and the electrically neutral neutron. Electrons, which carry a negative charge, occupy the space surrounding the nucleus.
The number of protons within the nucleus, known as the atomic number, defines the element; for example, every atom with six protons is carbon. Protons and neutrons possess nearly identical mass, approximately one atomic mass unit (amu) each. The electron is significantly lighter, weighing only about 1/1836th the mass of a proton, meaning its contribution to the atom’s total mass is negligible.
While the number of protons defines the element, the number of neutrons can vary, leading to different forms of the same element called isotopes. These isotopes have different mass numbers but maintain nearly identical chemical properties because their number of protons and electrons remains the same. The balance between the positive charge of the protons and the negative charge of the electrons ensures that the atom remains electrically neutral.
Elementary Particles and the Standard Model
Protons and neutrons, once thought to be fundamental, are composite particles known as hadrons. They are composed of even smaller elementary particles called quarks, which, alongside leptons, are the building blocks of matter in the Standard Model of particle physics. The familiar proton is made of two “up” quarks and one “down” quark, while the neutron consists of one “up” quark and two “down” quarks.
Quarks and leptons are grouped into two families of six particles each. The six quarks (up, down, charm, strange, top, and bottom) carry fractional electric charges, such as positive two-thirds or negative one-third of the elementary charge. The six leptons include the electron, two heavier versions (the muon and the tau), and three types of neutrinos, which are electrically neutral and possess small mass. All stable matter in the universe is made only from the lightest generation: the up quark, the down quark, and the electron.
These matter particles interact through fundamental forces mediated by force-carrying particles called bosons. The strong nuclear force, carried by the gluon, binds quarks together inside the nucleus. The photon carries the electromagnetic force, responsible for light and chemical bonding between atoms. The weak nuclear force, responsible for radioactive decay, is mediated by the W and Z bosons. The Higgs boson is associated with a field that pervades space, giving other elementary particles their mass.
The Mysterious Majority
Despite the complexity of the Standard Model, all the quarks, leptons, and bosons described only account for a small fraction of the universe’s total mass and energy. Everything we can observe—stars, planets, and ourselves—comprises only about 5% of the total mass-energy content of the cosmos. The majority of the universe is composed of components that do not fit into the established particle physics framework.
The next largest component is dark matter, which constitutes approximately 27% of the universe. This substance does not emit, absorb, or reflect light, making it invisible to telescopes. Its presence is inferred through its strong gravitational influence on visible galaxies and galaxy clusters. Scientists hypothesize it is made of some form of yet-undiscovered subatomic particle that interacts very weakly with ordinary matter.
The largest component, at roughly 68%, is dark energy, which drives the accelerating expansion of the universe. Dark energy is not a form of matter but a mysterious force or property of space itself that counteracts gravity on cosmological scales. Its nature is poorly understood, representing a major challenge in modern physics.