Will we ever… understand what dark matter is made from...
Perhaps the biggest secret in the Universe is the mysterious substance that makes up most of its mass. If dark matter does exist, what exactly is it?
What Is Dark Energy?
This diagram reveals changes in the rate of expansion
since the universe's birth 15 billion years ago. The more shallow the curve, the
faster the rate of expansion. The curve changes noticeably about 7.5 billion
years ago, when objects in the universe began flying apart as a faster rate.
Astronomers theorize that the faster expansion rate is due to a mysterious, dark
force that is pulling galaxies apart.
More is unknown than is
known. We know how much dark energy there is because we know how it affects the
Universe's expansion. Other than that, it is a complete mystery. But it is an
important mystery. It turns out that roughly
68% of the Universe is dark energy. Dark matter makes up about 27%. The rest
- everything on Earth, everything ever observed with all of our instruments, all
normal matter - adds up to less than 5% of the Universe. Come to think of it,
maybe it shouldn't be called "normal" matter at all, since it is such a small
fraction of the Universe.
One explanation for dark
energy is that it is a property of space. Albert Einstein was the first person
to realize that empty space is not nothing. Space has amazing properties, many
of which are just beginning to be understood.
The first property that Einstein
discovered is that it is possible for more space to come into existence. Then
one version of Einstein's gravity theory, the version that contains a cosmological
constant, makes a second prediction: "empty space" can possess its own
energy. Because this energy is a property of space itself, it would not be
diluted as space expands. As more space comes into existence, more of this
energy-of-space would appear. As a result, this form of energy would cause the
Universe to expand faster and faster. Unfortunately, no one understands why the
cosmological constant should even be there, much less why it would have exactly
the right value to cause the observed acceleration of the Universe.
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