Most of us have heard of mysterious, so-called dark matter lurking somewhere among the stars in the universe. If asked to describe what dark matter is in 25 words or less, we'd probably fail the test - myself included. Understanding the idea of dark matter involves complex physics and mathematics. So, for you and me, we'll just have to take the word of the experts on faith.
What exactly is the dark matter astrophysicists and astronomers have been abuzz about since the 1980s?
First, our current understanding of the material composition of the cosmos - and this is based on both advanced mathematics, direct observation, and extrapolation - breaks down thus: 70 percent dark energy, 25 percent dark matter, and 5 percent normal matter (the stuff you and I are made of).
As you can see, there's a lot of dark matter out there - a lot more than normal matter. So where is all this stuff? And is any of it on or near the Earth and is it dangerous?
NASA scientist Paul Hertz likes to say, "We are much more certain of what dark matter is not than we are what it is."
Let's go along with Hertz's reasoning and see, first, what dark matter is not. Perhaps by the process of elimination we can come closer to understanding what this stuff really is.
•According to Hertz, dark matter is - well - dark. This means it does not shine of its own (thermonuclear fusion like the Sun) or reflect photons/light (like the Moon). It is not in the familiar form of stellar and quasi-stellar objects we see with our naked eye or through a telescope, such as stars and planets, asteroids, comets, etc.
Hertz noted that Earth- and space-based observations reveal that there is just too little visible stuff (normal matter) in the cosmos, so you can't explain the "missing" 25 percent of matter by simply shrugging your shoulders and saying, "Oops, I guess we can't see all the dark clouds composed of normal matter in space, that is, the normal matter made up of particles called baryons. So this must be what we imagine to be dark matter."
Baryons are the familiar subatomic stuff you and I - and all that we see - are composed of. Unfortunately, dark clouds in space are too easy of an excuse to explain away the mysterious missing 25 percent of cosmic matter.
Fact: Dark matter is not simply dark, normal clouds of matter in space.
•The reasons astronomers dismiss the dark clouds in space idea is because they can easily demonstrate that delicate instruments can detect baryonic clouds (again, the stuff of normal matter) by the absorption of cosmic radiation passing through them.
Fact: Dark matter is non-baryonic matter.
• Let's clear up a common misconception about dark matter: dark matter is not antimatter. When observing the missing 25 percent, astronomers do not see gamma rays typically produced when antimatter annihilates with matter. Antimatter is pretty cool stuff by itself, but it's clearly not dark matter.
Fact: Dark matter is not antimatter.
•So, what about those large galaxy-sized black holes like the one lurking at the very core of our Milky Way galaxy?
"High concentrations of matter (called gravitational lenses) bend light passing near them from objects further away, but we do not see enough lensing events to suggest that such objects to make up the required 25 percent dark matter contribution," Hertz said.
Fact: Dark matter does not make up black holes, big or small.
Are you getting impatient yet?
"Yes, yes," I hear you say. "I get the idea. I now know what dark matter is not - so please, please tell me what exactly dark matter is?!"
•Baryonic matter (like you and me and all that we see) might end up being the culprit behind dark matter after all - non-baryonic dark matter could be tied up inside baryonic brown dwarfs, space objects betwixt and between planets and stars, or in dense chunks of unusually heavy chemical elements known as MAssive Compact Halo Objects or MACHOs.
TBD (to be determined): Dark matter might be composed of MACHOs hiding within dense normal-matter brown dwarfs or dark matter might also be in chunks of stable MACHO elements.
•Here's another theory of what dark matter is - it might be non-baryonic matter made up of exotic particles such as axions or hypothetical WIMPS (Weakly Interacting Massive Particles).
Fact: After all that, in the end, astronomers still don't know what dark matter is.
So far, dark matter has not been found on Earth or anywhere nearby. If it doesn't interact with normal matter, or if if it peacefully coexists with baryonic matter, it probably is not dangerous to life. But we don't know for sure. The whereabouts of dark matter remains among the cosmos' most intriguing mysteries.
Lou Varricchio, M.Sc., was a former senior science writer at the NASA Ames Research Center in California. His science reports have been heard and seen on public radio and television. He is currently a member of the NASA-JPL Solar System Ambassador program. He was a 2010 recipient of the U.S. Civil Air Patrol's Gen. Charles "Chuck" Yeager Aerospace Education Achievement Award.