Dark matter is thought to be invisible and it is also believed that most everything in the universe is made up of dark matter. It does not produce enough light that we can detect but there are other ways that we can detect it. Dark matter is thought to have a gravitational pull on light and sources of light and it is that extra gravitational pull that we can detect. From this extra gravity that we observe we can theorize how much mass is present.
All of this is of course is theory and scientists are working very hard to figure out exactly what dark matter is and what importance it holds in the universe.
If I could answer that, I'd be booking my place for an airplane flight to Stockholm for the next Nobel Prize presentations (if the volcano would let me fly there).
The reason I say this is because no one knows what dark matter is. The scientists cannot really even prove that it exists in any way except by inference. The only evidence for dark matter is the fact that various things in space (like clusters of galaxies) is affected by gravity more than it should be given how much matter can be seen.
This implies that there is some other stuff out there that we can't see. Scientists have named this stuff dark matter and they believe in makes up about 23% of the universe.
Dark matter is the invisble clustered galaxies in the universe. The term was initally created by Fritz Zwicky in the 1930s.
- We are much more certain what dark matter is not than we are what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the Universe to make up the 25% required by the observations. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter. Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. High concentrations of matter bend light passing near them from objects further away, but we do not see enough lensing events to suggest that such objects to make up the required 25% dark matter contribution.
However, at this point, there are still a few dark matter possibilities that are viable. Baryonic matter could still make up the dark matter if it were all tied up in brown dwarfs or in small, dense chunks of heavy elements. These possibilities are known as massive compact halo objects, or "MACHOs". But the most common view is that dark matter is not baryonic at all, but that it is made up of other, more exotic particles like axions or WIMPS (Weakly Interacting Massive Particles).
Dark matter in astronomy is the name give to an invisible matter or substance of unknown composition, which is supposed to constitute more than nine tenths of the total mass of galaxies and clusters of galaxies. This matter, unlike all other matters or substances, does not give off, reflect, or absorb light. This is the reason why it is called dark matter.
Scientists have proposed different theories or hypothesis about the nature of the dark matter but there none of theories is are considered to be satisfactory explanations of nature of dark matter. The nature of dark matter is one of the most important unsolved mystery in science today.
Dark matter should not be confused with black holes which are collapsed objects, such as a stars, that have become invisible. They has such a strong gravitational force that even light is pulled in by it. Black holes are invisible because any light approaching them is pulled in by the gravitational pull and no light can then escape away from it.
Stars are bright matter, while planets are dull matter. Dark matter refers to gravitational effects that cannot be attributed to specific seen matter, such as a star or a planet. Because we don't "see" the matter, it is referred to as "dark" matter. Which is really no big deal, except that some say dark matter accounts for a lot of the gravitational effects -- that is, there is a lot of dark matter. Number one guess -- dust of the universe; of course, that's a spread-out distribution. I'm not up on the distribution of dark matter.
I should note that energy, even light, would have a matter signature -- that is, acts like mass (E=M*C*C). So if a laser light were to pass by some matter, the matter would be gravitationally drawn to the light, even though it couldn't see the light (because the light was directed somewhere else).
I no longer believe in the existence of dark matter (as a particular substance). I believe that the physics of our galaxy, and greater cosmology, can be explained with an adapted version of Einstein's Theory of General Relativity (I have been calling it Analytically Extended General Relativity.) It leads straight to a quantum theory of gravity. www.hugsleftbrain.com