Not all Matter is Visible
Our galaxy, the Milky Way, is built up of billions of star, all traveling around in their orbits. Scientists can calculate the total gravitational force in our galaxy by measuring the velocity and mass of the outer most stars in the galaxy. They found that the velocities of the stars would cause them to leave their orbits and continue to travel in space. Our galaxy's gravitation force that keeps the stars in orbit is much stronger than they calculated.
This is resolved by a theory many scientists have come to accept as truth, that our galaxy must contain ten times the matter than that which we can see, called dark matter. Dark matter is not visible because it neither emits nor reflects electromagnetic radiation and scientists have not been able to come up with a way to detect it. About 90% of all the mass in the universe is believed to be made up by this 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.
MACHO's and WIMP's
Scientists have tried to identify dark matter in various ways, it seems now that ordinary matter have been ruled out as the primary component of dark matter. Some scientists think it might be remnants of planets and dead stars, or clouds of gases and dust where star formation has not yet taken place. Two theories have prevailed that explain just what dark matter is made of.
Some theories state that dark matter may be 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.
Other theories believe 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 particles are thought to be undiscovered yet and not made out of protons and neutrons like our own elements. They are electrically neutral and pretty much invisible, so that is why we can not detect them. Also, these particles would be very tiny, perhaps something like one hundred thousand times smaller than an electron. That would account for why they have not been discovered.