What is dark matter, and what are the problems with it?

Dark matter may sound very mysterious, but it is simply a name which astronomers give to any stuff in the universe which we can detect gravitationally but not "see". In other words, for many possible reasons, the material is not emitting light of any wavelength which we can detect, but we can measure its gravitational effect on other objects in the universe.

The first evidence of dark matter was found in clusters of galaxies back in the 1930's. Astronomer Fritz Zwicky discovered that the mass of luminous material in a cluster of galaxies (i.e. the galaxies themselves and any gas which was detected) was much less than the total mass of the cluster implied by the velocities of the galaxies. Evidence of dark matter has since been found within galaxies, and it appears that more than 90% of the total mass of the universe may be dark matter.

The search for dark matter continues. Some dark matter is in the form of 'brown dwarfs', 'black dwarfs', and planets, which we know exist but which are generally too faint to be detected other than by their gravitational effect. It seems unlikely, however, that these dim objects can account for all of the missing mass. There are many other candidates, ranging from as yet undetected exotic particles to black holes. Stay tuned to 'The Universe' for more on dark matter.

Submitted by Ron (Israel)
(March 13, 1998)

You might also be interested in:

What are the retrograde motions of planets in the sky?

It depends on what kind of motion you are talking about. When seen from the north pole of the celestial sphere all planets orbit around the Sun in a counter-clockwise or direct path. Most planets also...more

How do Astronauts Live in Space?

Almost everyone has a question or two about living in space. What do astronauts do in space? How do they do everyday things like eat, sleep and go to the bathroom? Well, this is our attempt to answer...more

How far is the Earth from the Sun, the Moon and all the other planets? How far are all of the planets from the Sun? Do you know of a software that tracks the planets in real-time?

There is a really neat internet program called Solar System Live that shows the position of all of the planets and the Sun for any given day. If you go to that page, you'll see an image similar to the...more

Is it really true that man never really walked on the Moon?

The picture of the American Flag (the one put there by the Apollo astronauts) is waving (or straight out) in the wind. How could that be possible if there is no atmosphere on the Moon? Was it some sort...more

How many planets orbit the sun?

I was wondering if there is a new planet? Are there planets (a tenth planet?) after Pluto belonging to our solar system? What are the names of the new planets discovered in the solar system? Are there...more

According to Stephen Hawking, any object with an energy which equals Plank's energy has to become a black hole.

If that is so, the energy released during the Big Bang must have created many such black holes. Therefore most of the Energy of the Big bang must have disappeared in that form. Then how did the Universe...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA