What is the universe made of? This is one of the most basic questions humanity has had about the world surrounding us, but has proven one of the most difficult to answer. Our knowledge of matter and energy has progressed rapidly over the last few centuries, leading to a detailed understanding of nearly all of the materials and phenomena around us. Scientists have even begun to understand the principles that have shaped the evolution of our universe itself over the past few billion years.
One of the most shocking discoveries of recent science, however, is that we have much yet to learn. Only a tiny fraction of the universe seems to be composed of the protons, neutrons, and electrons we have labored so long to understand. The identity of the vast majority of the cosmos remains a mystery!
Our present understanding of the universe's composition is roughly as follows:
Baryonic Matter: ~5% of the mass in the universe
This is ordinary matter composed of protons, neutrons, and electrons. It comprises gas, dust, stars, planets, people, etc.
Cold Dark Matter: ~25%
This is the so-called "missing mass" of the universe. It comprises the dark matter halos that surround galaxies and galaxy clusters, and aids in the formation of structure in the universe. The dark matter is said to be "cold" because it is nonrelativistic (slow-moving) during the era of structure formation. Dark matter is currently believed to be composed of some kind of new elementary particle, usually referred to as a weakly interacting massive particle (WIMP).
Dark Energy: ~70%
Through observations of distant supernovae, two research groups have independently discovered that the expansion of the universe appears to be getting faster with time. This seems to require some kind of "antigravity" effect which we do not understand. Cosmologists believe that the acceleration may be caused by some kind of new energy field that permeates the universe, perhaps even the cosmological constant that Einstein imagined almost a century ago. Whatever the source of this phenomenon turns out to be, cosmologists refer to it generically as dark energy.
The focus of these pages is on the second of these components - cold dark matter. We hope to give an idea of why we think this bizarre stuff is out there, what it might be, and how we hope to unlock its mysteries. Follow the links at left for further information!