For centuries, humanity has pondered the fundamental composition of the cosmos, driving profound scientific inquiries. The journey to understand what the universe is made of has moved from simple philosophical ideas to complex, layered scientific models, revealing a reality far stranger than once imagined. The vast majority of existence is composed of invisible forces and particles that defy our everyday understanding of matter, and our current cosmological inventory is the result of a historical progression where each discovery built upon the last.
Defining the Visible Universe: Atoms and Elements
The earliest attempt to define the universe’s material came from Greek thinkers like Democritus, who, around the 4th century B.C., proposed that all matter consisted of indivisible particles he called atomos. This philosophical concept remained theoretical until the early 19th century when John Dalton established the first modern atomic theory based on empirical evidence. Dalton proposed that each element was made of a distinct type of atom with a characteristic mass, moving the discussion from philosophy to quantitative science.
Building on this foundation, Dmitri Mendeleev organized the known elements in 1869 according to increasing atomic weight and recurring chemical properties. Mendeleev’s periodic table systematized the known building blocks and successfully predicted the existence and properties of undiscovered elements, solidifying the atom as the foundational unit of all observable matter. The familiar matter that forms stars, planets, and life is known as baryonic matter, which accounts for only about 4.9% of the total mass-energy of the universe. This small fraction represents the entirety of the “visible” universe.
The Standard Model of Fundamental Particles
While Dalton established the atom as the chemical unit, 20th-century physics revealed that atoms are not fundamental but are constructed from smaller, elementary particles. This deeper understanding is captured in the Standard Model of particle physics, which classifies all known matter particles and the forces governing their interactions. Matter particles are divided into two main groups: quarks, which combine to form protons and neutrons, and leptons, which include the electron.
The existence of quarks was proposed independently by Murray Gell-Mann and George Zweig in 1964. The Standard Model also describes the exchange of force-carrying particles, or bosons, such as the photon (mediating the electromagnetic force) and the gluon (binding quarks together). The final piece of the model, the Higgs boson, was confirmed in 2012, providing a mechanism that explains why these fundamental particles possess mass.
Unmasking the Invisible Mass: Dark Matter
The first suggestion that the universe contained a significant amount of unseen mass came in 1933 from astronomer Fritz Zwicky, who studied the movement of galaxies within the Coma Cluster. Zwicky observed that the galaxies were moving so quickly that the cluster’s visible mass did not provide enough gravitational pull to keep them bound together. He concluded that a substantial amount of non-luminous, or “dark,” matter must be present to supply the necessary gravity.
Decades later, in the 1960s and 1970s, astronomer Vera Rubin and her colleague Kent Ford provided the definitive observational evidence for this missing mass by studying the rotation of spiral galaxies. They found that stars far from the galactic center were orbiting just as fast as those closer in. This contradicted expectations, as the gravitational pull should have weakened with distance from the visible mass. Rubin’s work showed that galaxies must be enveloped in a vast, invisible halo of dark matter. Dark matter is estimated to constitute about 26.8% of the total mass-energy of the universe, providing the gravitational scaffolding that allows galaxies and clusters to form.
The Driving Force of Expansion: Dark Energy
The most surprising and dominant component of the universe was discovered in the late 1990s by two independent teams of astronomers, led by Saul Perlmutter, and by Brian Schmidt and Adam Riess. These teams observed distant Type Ia supernovae, which serve as extremely reliable “standard candles” for measuring cosmic distances. By comparing the known intrinsic brightness of these stellar explosions with their observed apparent brightness, the scientists could accurately gauge their distance from Earth.
The expectation was that the universe’s expansion, initiated by the Big Bang, should be slowing down due to the collective gravitational pull of all matter. Instead, the distant supernovae appeared fainter than they should have been, indicating they were farther away than predicted for a decelerating universe. This observation led to the astonishing conclusion that the expansion of the universe is actually accelerating. This acceleration is attributed to a mysterious force or energy density permeating all of space, which was named dark energy. Dark energy is the largest single constituent of the cosmos, accounting for an estimated 68.3% of the total mass-energy budget.