Why Dark Matter
Orbital MechanicsGravitational LensingBig Bang NucleosynthesisThe Cosmic Microwave BackgroundLarge Scale StructureModified Gravity

Why Dark Matter?

Every object you've ever seen in your entire life - your TV, your books, your own body - is built up of three simple ingredients: protons, neutrons, and electrons. These three particles even make up the objects we see shining in the daytime and nighttime skies - the sun, the moon, the planets, and the stars. In fact, they are the primary constituents of all visible matter.

Modern cosmology and astrophysics, however, tell us something pretty remarkable - these three particles are not the main constituents of the universe we live in! The universe as a whole is made almost entirely out of mysterious stuff that no one has ever seen before, stuff that we call "dark matter" and "dark energy". The protons, neutrons, and electrons that dominate the world we see around us are just the glittery icing on a vast universe of darkness.

Our current picture of the "energy budget" of the universe is described in the pie chart at left. About 70% of the mass of the universe is believed to be composed of "dark energy", a mysterious substance or energy field that seems to permeate the universe, causing its expansion to speed up over time. Something like 25% is composed of "dark matter" - some sort of stuff that (like ordinary matter) clumps together under its own gravity, but is somehow invisible to us. Finally, the remaining 5% or so is ordinary matter - stars, planets, gas, dust, and all the rest.

This sounds like a crazy idea, but there are actually many good reasons to believe that it's true! See below for a tour of some of the arguments that have convinced many scientists that the universe is more than meets the eye.

Mapping Matter

One way to look for dark matter is to "simply" count up the total amount of matter and compare it to the total amount of visible matter - if the two don't agree, their must be dark matter! Astronomers can "count" the amount of visible matter using the total amount of light coming from the galaxy (this isn't as easy as it sounds, but we won't get into that here). To do the comparison, you need to find some way to independently estimate the total amount of matter, visible and invisible. Two major methods are described below.

Galaxy rotation curves and other motions in the universe - what can you learn about invisible matter by watching how visible objects move?

Can we map the distribution of matter - both visible and invisible - by studying how its gravity bends the light from distant sources?

The Universe's Baby Pictures

Another way to study the amount of dark matter is to study its effects on the universe's early history and evolution. By finding windows onto the state of the universe shortly after its birth, we can study how it has grown and changed since then. Dark matter has had major effects on the development and bahavior of the universe, and by studying these phenomena we can estimate how much dark matter there is and - perhaps more importantly - what properties it might have.

How much of each kind of chemical element is there, and what does this tell us about the conditions in the early universe?

What can the residual "afterglow" of the Big Bang itself tell us about the composition of the universe?

What effect does dark matter have on the arrangement of galaxies?

Could We Be Wrong?

In science we must always consider the possibility that we are wrong. Dark matter explains our observations exceedingly well, but it's important to consider other options and devise ways of comparing their predictions to those of dark matter theories.

Could the equations of gravitation be modified so that dark matter is no longer required?