However, scientists think that matter that emits light only makes up a small part of the
matter in the universe. The rest of the matter is calleddark matter.
Because dark matter doesn’t emit light, we can’t observe it directly. However, we know
it is there because its gravity affects the motion of objects around it. For example, when
astronomers measure how spiral galaxies rotate, they find that the outside edges of a galaxy
rotate at the same speed as parts closer to the center. This can only be explained if there is
a lot of extra matter in a galaxy that we cannot see.
So what is dark matter? Actually, we don’t really know. One possibility is that it could just
be ordinary matter—protons, neutrons, and electrons, like what makes up the Earth and all
the matter around us. The universe could contain lots of objects that don’t have enough
mass to glow on their own, such as large planets andbrown dwarfs, objects larger than
Jupiter but smaller than the smallest stars. Or, there could be large numbers of undetected
black holes.
Another possibility is that the universe contains a lot of matter that is unlike anything we
have ever encountered. For example, scientists have proposed that there might be particles
that have mass but don’t interact much with other matter. Scientists call these theoretical
particles WIMPs, which stands for Weakly Interactive Massive Particles. WIMPs would
have a gravitational effect on other matter because of their mass. But because they don’t
interact much with ordinary matter, they would be very difficult or impossible to detect
directly.
Most scientists who study dark matter believe that the universe’s dark matter is a combi-
nation of ordinary matter and some kind of exotic matter that we haven’t discovered yet.
Most scientists also think that ordinary matter is much less than half of the total matter
in the universe. Researching dark matter is clearly an active area of scientific research, and
astronomers’ knowledge about dark matter changing rapidly.
Dark Energy
Astronomers who study the expansion of the universe are interested in finding out just how
fast the universe is expanding. For years, the big question was whether the universe was
expanding fast enough to overcome the attractive pull of gravity. If yes, then the universe
would expand forever, although the expansion would slow down over time. If no, then the
universe would someday start to contract, and eventually would get squeezed together in a
big crunch, the opposite of the Big Bang.
Recently, however, these astronomers have made a strange discovery: the rate at which the
universe is expanding is actually increasing. In other words, the universe is expanding faster
now than ever before, and in the future it will expand even faster! This answers the old
question: the universe will keep expanding forever. But it also proposes a perplexing new
question: what is causing the expansion of the universe to accelerate?