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The Question

(Submitted January 04, 2001)

What is the difference between Dark Energy and Dark Matter; aren't they considered the same thing?

The Answer

Dark Energy appears to be, based on the brightness of the most distant type-Ia supernovae, a mysterious force that is accelerating the expansion of the universe. These recent discoveries have provided good evidence that there is such an outward force on the universe (variously called the cosmological constant, "quintessence," or "dark energy").

Data about the rotation of galaxies shows us that the outer parts rotate as fast as the inner parts. This only makes sense if there is a spherical distribution of matter in each galaxy, which is not what we see. Therefore we infer that there is a certain amount of Dark Matter in each galaxy. This could be some exotic particles, or just lots of stars too small to have ignited.

Aside from this, there is also the Dark Matter that we think is there, based on theoretical arguments. This is something we can measure by looking at the cosmic microwave background and distant supernovae. These are the measurements (recently made) that imply the existence of both Dark Matter and Dark Energy.

You can read a bit about these recent results at

And more detail than you probably want at

Hope that helps.

-Kevin Boyce and Martin Still,
for "Ask an Astrophysicist"

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