Dark matter and dark energy
are two important concepts in modern astronomy, and both concern
the same problem. When scientists and astronomers attempt to calculate
how much mass should exist in the universe, and compare it with
the amount of matter they can see, they find that only a miniscule
fraction of what is there is actually visible. What astronomers
can see in the sky accounts for very little of the matter that
calculations suggest is actually there. Dark matter is the title
given to the objects which represent the ‘missing matter’.
Dark matter could be any number of things; black holes are a popular
example. They are very dense points of matter and have extremely
high masses, but don’t give off any visible light and hence can’t
be observed. Dark matter can be inferred, however, because any
matter that exists in space exerts an effect on the motion of
nearby objects through its gravity.
Dark energy is another possible explanation for this ‘missing
mass’. Scientists have shown that the universe is expanding, and
has been since the big bang. However, this contradicts fundamental
assumptions about gravity; for the universe to expand, everything
needs to be moving away from everything else, whereas objects
with gravity should be pulled closer together. Basic physics tells
us that for this expansion to occur, there must be a force pushing
things out which counteracts gravitational pull. Scientists refer
to this pushing force as ‘dark energy’.
The predominant theories about the nature of dark energy is known
as the cosmological constant; it states that the amount of dark
energy in space is a constant, and that it has a low, even density.
This means that any volume of space has some intrinsic amount
of energy in it; and since Einstein’s relativity theory has shown
that mass and energy are related, this energy has mass, which
means it has gravitational effects. Its effect is negative, however;
it exists in a vacuum, and hence exerts an anti-gravity force.
This anti-gravity force would drive the expansion of the universe,
and simultaneously account for much of the ‘missing mass’.
However, scientists have by no means reached an agreement on the
nature of either dark matter or dark energy. There are many competing
theories, all of which closely intertwine with advanced concepts
in astrophysics and particle physics. Trying to determine the
nature of dark energy is one of the biggest efforts in space science
today.
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