Brown
Particle Astrophysics Group

Dark Matter

What is Dark Matter?

The concept of dark matter arose as a solution to a problem that has been puzzling astronomers for decades. Galaxies, when observed, rotate at a much faster rate than is expected for the estimated mass they contain. Astronomers and physicists have suggested that the extra mass may be accounted for by dim objects that our instruments cannot detect, or that the laws of gravitation are different over large distances. The most convincing argument, however, is that a previously unknown type of matter—dark matter—is clumped throughout the universe. We did not know about dark matter before now because it rarely interacts with regular (what physicists call baryonic) matter or with light other than through gravitation.

So what’s it made of?

Because of its relatively inert nature, the physical makeup of dark matter remains a mystery. Some likely candidates are Massive Compact Halo Objects (MACHOs) and Weakly Interacting Massive Particles (WIMPs), each of which describes a number of theoretical particles. Our research is designed to detect the latter of these possibilities. Right now, the most probable candidate for a dark matter WIMP is a particle called the neutralino. The neutralino is one of the particles whose existence is predicted by the theory of supersymmetry, which attempts to unite the four natural forces under a single theory.

How much of this stuff are we talking about?

According to the most recent studies, the universe is 4% baryonic matter, 23% dark matter, and 73% dark energy. Baryonic matter consists of the atoms of which you and everything you interact with is made. This includes stars, galaxies, planets, and just about everything else astronomers can visually observe. Actually, less than one percent of the universe can be seen with current technology. The other three percent of baryonic matter, along with the percentages of dark matter and dark energy, are inferred from observations of the cosmic microwave background, supernovae, and other indicators.

Wait—dark energy?

Yes, dark energy. Dark energy is a concept developed only in the past couple of years to describe the intrinsic expansive energy associated with a vacuum. This is Einstein’s infamous “cosmological constant” which keeps the universe from collapsing upon itself under its own gravity. The amount of dark energy is directly dependent upon the volume taken up by vacuum in the universe. This means that as the universe expands, dark energy will become more and more abundant. Since it is a repulsive force, an increase in dark energy will lead to an acceleration in the universe’s expansion.

What is the fate of the universe?

Because of dark energy’s increasing influence over time, our universe is predicted to expand into eternity. Eventually the stars will die out and all that will be left are black holes and clumps of matter. Boring, yes, but a fitting end nonetheless.

So why should we care what dark matter is?

Science began as the human quest for knowledge and understanding. On a short-term scale, scientific research tells us what, when, where, and how. But as part of the big picture, the purpose of scientific inquiry is and always has been to answer the biggest question of all: why? Insight about dark matter may provide us with an answer to this question. Once we know what constitutes 85% of the matter in the universe, we will be able to more deeply understand its origins. Only then will we be able to comprehend our own.