Since the early 20th century, astronomers and physicists have known that the universe contains more than meets the eye. Through observations of the motions of galaxies and the behavior of light, scientists have determined that the majority of the universe is composed of substances we cannot directly observe - dark matter and dark energy.
Dark Matter
The existence of dark matter was first proposed in the 1930s by Swiss astronomer Fritz Zwicky. He noticed that galaxies in galaxy clusters were moving much faster than expected based on the amount of visible matter they contained. This implied that there must be some unseen, undetectable "dark" matter providing additional gravitational mass to hold the clusters together.
Further evidence for dark matter has accumulated over the decades. Measurements of the cosmic microwave background - the faint afterglow of the Big Bang - show that normal, baryonic matter (the stuff that makes up planets, stars, and galaxies) accounts for only about 5% of the total mass-energy content of the universe. The rest is dark matter.
Dark matter does not emit, reflect or absorb light, so it cannot be directly observed. But its gravitational effects can be detected. Galaxies and galaxy clusters act as gigantic gravitational lenses, bending and distorting the light from background objects in predictable ways. By mapping these distortions, astronomers can infer the distribution of dark matter.
The leading theory is that dark matter is composed of as-yet-undiscovered subatomic particles that do not interact with light. Potential dark matter particle candidates include weakly interacting massive particles (WIMPs) and axions. Experiments deep underground and at particle accelerators are searching for these elusive particles.
Dark Energy
In the late 1990s, observations of distant supernovae revealed an even more perplexing cosmic phenomenon - the expansion of the universe is accelerating. This acceleration implies the existence of a mysterious "dark energy" that is driving space itself to expand at an ever-faster rate.
Dark energy makes up about 70% of the total mass-energy content of the universe. Its nature is even more enigmatic than dark matter. The leading explanation is that it is a property of space itself - a kind of repulsive "cosmic antigravity" that pushes space to expand. This idea is based on Einstein's theory of general relativity, which allows for the existence of such a "cosmological constant."
Other proposed explanations for dark energy include modifications to general relativity or the existence of a new, unknown quantum field permeating all of space. But the true nature of dark energy remains one of the biggest unsolved mysteries in physics.
The Unseen Majority
Together, dark matter and dark energy make up 95% of the universe. The remaining 5% is the familiar stuff of planets, stars, and galaxies - the world we can observe and interact with directly.
The discoveries of dark matter and dark energy have radically reshaped our understanding of the cosmos. They tell us that the universe is far stranger and more complex than the visible matter we can observe. Unraveling their mysteries is one of the greatest challenges facing 21st century physics and cosmology.
Ongoing and future experiments and observations, from the Large Hadron Collider to the next generation of powerful telescopes, will continue to shed light on these invisible cosmic components. Only then can we truly understand the full sweep of the universe we inhabit.
Tags:
Science & Technology