The universe is a vast expanse, but what it is truly made of remains largely a mystery. According to the standard model of cosmology, less than 5 percent of the cosmos consists of ordinary matter—the familiar atoms making up everything from stars and planets to people. The vast majority, a staggering 95.1 percent, is composed of elusive dark matter and mysterious dark energy, forces that scientists still struggle to comprehend. Current estimates pinpoint the universe's total mass-energy budget as 4.9 percent ordinary matter, 26.8 percent dark matter, and 68.3 percent dark energy. This means everything we can see, touch, and measure accounts for only a tiny fraction of existence.[spacedaily+2]
The Visible Universe: A Tiny Fraction
Ordinary matter, also known as baryonic matter, is the stuff of everyday life. It includes all the atoms on the periodic table: protons, neutrons, and electrons that form stars, planets, gas, dust, and all living organisms. Despite its familiarity, this "normal" matter makes up a remarkably small portion of the universe's total mass-energy. The 4.9 percent figure represents all the directly observable components of the cosmos.[spacedaily+8]
Even within this small slice, much of ordinary matter remains unseen. It includes cold gas, diffuse intergalactic material, dead stars, and hot ionized plasma. Stars and galaxies, which produce nearly all the light we observe, account for less than 10 percent of the ordinary matter in the universe. The majority of baryonic matter hides in the vast, less dense filaments of the cosmic web that connect galaxy clusters.[spacedaily+4]
The Enigma of Dark Matter
Dark matter constitutes a significant 26.8 percent of the universe's mass-energy budget, yet scientists have never directly observed it. It is "dark" because it does not absorb, reflect, or emit light, making it invisible to telescopes. Its existence is inferred solely through its gravitational effects on visible matter. Dark matter acts like an invisible scaffold, providing the extra gravitational pull needed to hold galaxies and galaxy clusters together.[spacedaily+11]
The concept of unseen matter dates back to the 1930s when Swiss-American astronomer Fritz Zwicky observed galaxies in the Coma Cluster moving much faster than expected based on their visible mass. Decades later, in the 1970s, American astronomer Vera Rubin found further evidence when studying the rotation curves of galaxies. She observed that stars at the edges of galaxies orbited just as fast as those closer to the center, implying a halo of invisible mass surrounding the galaxies. Without this extra gravitational force from dark matter, galaxies would simply tear themselves apart. Other evidence for dark matter comes from how light bends around massive objects, a phenomenon called gravitational lensing, and from patterns in the cosmic microwave background radiation.[energy+3]
Scientists believe dark matter is "cold," meaning its particles move slowly compared to the speed of light, allowing small structures to form and merge into larger ones. It is also "non-baryonic," meaning it is not made from the same protons and neutrons that form ordinary matter. Leading candidates for dark matter particles include Weakly Interacting Massive Particles (WIMPs) and axions, though none have been confirmed. Experiments like those at the Large Hadron Collider (LHC) search for signs of dark matter by looking for missing energy after particle collisions. Underground detectors, shielded from cosmic rays, also seek to catch rare interactions between dark matter particles and ordinary matter.[en+11]
The Accelerating Universe: Dark Energy
Dark energy makes up the largest component of the universe's mass-energy budget at 68.3 percent. It is the mysterious force responsible for the accelerating expansion of the universe. Unlike dark matter, which clumps with galaxies, dark energy is thought to be spread uniformly throughout space. Its primary effect is a repulsive gravitational force that pushes space outward.[spacedaily+14]
The discovery of dark energy in the late 1990s came as a major surprise to the scientific community. Astronomers studying distant Type Ia supernovae, which act as "standard candles" for measuring cosmic distances, expected to find that the universe's expansion was slowing down due to gravity. Instead, observations showed that the supernovae were fainter and farther away than predicted, indicating that the expansion was accelerating. This unexpected acceleration led to the concept of dark energy.[sci+7]
One of the leading explanations for dark energy is the "cosmological constant," a concept Albert Einstein first introduced into his theory of general relativity in 1917. He initially used it to balance gravity and maintain a static universe. After Edwin Hubble discovered the universe was expanding in the 1920s, Einstein reportedly called the cosmological constant his "biggest blunder". However, modern cosmology has reintroduced the cosmological constant as the simplest way to explain the observed cosmic acceleration. This theory suggests that dark energy is an intrinsic property of the vacuum of space itself, and its energy density remains constant even as the universe expands. Other theories, like "quintessence," propose that dark energy is a dynamic field that can change over time and space.[science+15]
Michael Turner, a University of Chicago astrophysicist who coined the term "dark energy" in 1998, emphasized its profound nature. "I think dark energy is the most profound mystery in all of science," Turner said. Understanding it is crucial for scientists to grasp the universe's beginning, evolution, and ultimate fate.[news]
The Standard Model and Future Exploration
The Lambda-CDM (Lambda-Cold Dark Matter) model is the current standard model of cosmology. It successfully explains many cosmological observations, including the cosmic microwave background radiation, the large-scale structure of galaxies, and the abundance of light elements. The precise percentages of ordinary matter, dark matter, and dark energy are derived from careful analysis of the universe's oldest light, the cosmic microwave background, by missions like the European Space Agency's Planck spacecraft. Planck mapped tiny temperature differences in this ancient radiation, providing crucial data about the early universe's density and motion.[en+7]
The dominance of dark matter and dark energy means that our understanding of the universe is still largely incomplete. Scientists are actively pursuing new research to unravel these cosmic mysteries. Upcoming missions, such as NASA's Nancy Grace Roman Space Telescope, are designed to explore dark energy and dark matter in greater depth. Dominic Benford, the program scientist for the Roman Space Telescope, highlighted the importance of this work. "Fundamentally, the physics of our universe is driven by the components that make up everything that we know of," Benford told Discover, noting that all visible matter comprises only about 5 percent of the total mass-energy budget.[roman+2]
The Dark Energy Spectroscopic Instrument (DESI) is also mapping the expansion history of the universe using millions of galaxies and quasars, providing further insights into dark energy's behavior. The European Space Agency's Euclid mission will precisely map the distribution of galaxies over 10 billion years of cosmic history to reveal how dark energy has accelerated the universe. These efforts aim to determine if dark energy is truly a constant force or if its influence changes over time. The quest to understand dark matter and dark energy continues to be a driving force in modern astrophysics, pushing the boundaries of our knowledge about the cosmos.[spacedaily+5]
