Dark matter
Planets stay in their orbit around the sun, stars turn around in their galaxy and galaxies are grouped together in clusters, due to gravitational forces. If gravity had no influence than these objects in motion would move away from each other. Gravity also determines the speed of the objects in their orbit. The farther away from the gravitational center, the slower the object moves.
The forces of gravity are determined by matter, everything remains in place if their is sufficient mass to generate the required gravitational forces.
We do not see all matter.
| Our galaxy, The Milkyway, is built up
of billions of star, all moving in orbits around the center. By measuring
the velocity and mass of the outer most stars of the Milkyway we can
calculate the total gravitational forces of the Milkyway galaxy. When you look at these
calculations , it appears that the velocities found are to large,
for these stars to stay in their orbits. They suggest that the planets should fly off into space.
The conclusion derived from this is that the gravitational force that 'holds' the stars in their orbits is much stronger than we have calculated. The total mass of the Milkyway must be ten times larger than the mass we can 'see'. Apparently we see only 10% of all matter! It is assumed that in the universe much more matter is present than we can account for, this matter is called dark matter. It is not readily visible because it neither emits nor reflects electromagnetic radiation and so far we have not figured out any other means to detect it. About 90% of all the mass in the universe is believed to be made up by this dark matter. |
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MACHO's and WIMP's.
Dark matter can consist of ordinary matter like we know it, made out
of atoms. It may be objects that emit too little light to observe, such
as remainders of planets and dead stars, or clouds of gases and dust where
star formation has not yet taken place.
It maybe made of something which is often called MACHO's, Massive
Astrophysical Compact Halo Objects. These are the dense and massive
remains of dead, burnt-out stars, red
dwarfs and/or black holes (large massive
objects, containing huge amounts of matter in a relatively small volume).
Other theories state that dark matter is made of elementary particles, possibly huge amounts of low mass neutrons, or what is called WIMP's, Weakly Interacting Massive Particles. These are particles that we have no knowledge of, it is believed that they are something not made of protons and neutron. They are electrically neutral and pretty much invisible, so we can not detect them. Also these particles would be very tiny, perhaps something like one hundred thousand times smaller than an electron, so small that they can not be measured.
Identifying dark matter.
Nobody knows exactly what dark matter is and how much of it can be
found in the universe. One thing is clear however, the solution of the
riddle of dark matter will give us a better idea of the structure and final
fate of the universe, since the amount of matter in it directly influences how long it will exist.
Scientists have tried to identify dark matter in various ways, it seems
now MACHO's, and other ordinary matter are to be ruled out as the primary
component of dark matter.