Why should I care about dark matter and energy?

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Answered by: Thomas, An Expert in the Space and Astronomy - General Category
Dark matter and energy are aspects of the universe that we can't see, feel, or use - yet.

Physics continues to improve its predictions of the way the world and the universe work, but physicists look to the borders of their knowledge and investigate the places where the predictions aren't accurate enough.

Within the last several years, predictions of the universe's growth suggested either a reversal, The Big Crunch; no growth, The Flat Universe; or slowing eternal expansion, The Big Chill. Instead what they observed was an acceleration. The universe is growing and the growth is accelerating. If the observations are correct then the predictions were wrong. What went wrong?

Gravity draws matter together. The Big Bang radically expanded the universe from nothing to what we see today. If there's enough matter, each bit of matter would pull on every other bit of matter until it all fell back into a dot, The Big Crunch. Anything less would result in a universe where gravity was never strong enough to slow down the bits that were flying apart. Eventually everything would drift far from its neighbors and cool down as the stars burned out, The Big Chill. The sweet and un-scary impossibility in the middle was just enough matter to slow everything and stop it. Space wouldn't curve into a tiny sphere nor would it let everything roll away, The Flat Universe.

Yet, the universe is flying apart, while some parts are gathered together in tighter lumps that gravity would allow. Dark matter and energy may be the reasons.

The universe is flying apart as if more energy was available. Each galaxy though seems to be tighter than it should be, as if there was extra matter creating extra gravity. Welcome to dark matter and dark energy. The energy is expanding the universe and carrying the galaxies with it. The galaxies contain enough matter to maintain their shape. That energy and that matter are directly visible though, and that's why they are called "dark".

If the hypotheses are correct, dark matter and energy account for more of the universe than the energy and matter that we're familiar with. It is a very humbling thought akin to the realization that the center of the universe isn't the Earth, nor the Sun, nor the Milky Way. The physics of normal matter and energy are powerful predictors of what happens in our normal lives, but only describe a small fraction of the universe.

When Newton published his equations for gravity, he radically improved the prediction of the movement of the planets, but he didn't have an explanation. He didn't know what gravity was. He only knew what it did. Einstein's predictions were substantiated when he predicted that light would be bent while passing matter, and others measured the effect by watching starlight passing our sun. Newton's equations didn't change the lives of most people, expect that he revolutionized physics and helped initiate the Industrial Revolution. Einstein's predictions of light bending didn't change the way street lights work, but it enabled the Atomic Age, and subsequently the Information Age as electrical engineers developed computers.

Understanding dark matter and energy is important, but it isn't only academic. We don't know if we can tap them, if any of their effects could explain parts of our lives; but the possibility exists that the universe is pervaded with more energy than we know. Perhaps we've finally found an inexhaustible energy source that has lived within our environment forever.

Dark matter and energy are great unknowns. Observations are beginning to corroborate predictions based on their hypothetical properties, but other physicists are re-examining the assumptions, the observations, and the equations. It is a fascinating story and we get to watch. Stay tuned.

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