Ask an astronomer what the universe is made of. He or she will describe stars and planets, giant clouds of gas and dust called nebulae and the variations and combinations of these, like star clusters, galaxies, black holes and quasars.
If your astronomer friend is fastidiously detailed, he or she will also mention all forms of electromagnetic radiation — radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays — and a collection of subatomic particles like neutrinos.
If you stop there, the astronomer will only be 20 percent correct.
All the matter made of electrons, protons and neutrons and all the known subatomic particles accounts only for 20 percent of the total matter in the universe.
The other 80 percent is made up of stuff that we have little understanding of, so little that we simply call it “dark matter.”
We can't see it.
It doesn't interact with ordinary matter in ways that we currently understand, but we know it's there because we can easily measure the effects of its gravitational pull on all the stuff that we can see.
According to a post at Technology Review's Physics arXiv blog, we're all getting popped by this stuff pretty regularly.
An average-sized human, a “70 kilogram lump of meat made largely of oxygen, hydrogen, carbon and nitrogen,” may get smacked quite often.
The researchers, Katherine Freese at the University of Michigan in Ann Arbor, and Christopher Savage at Stockholm University in Sweden, calculate that there may be some 100,000 hits per year for each human on the planet. That's about once every five minutes per person.
Freese and Savage don't make any claims about the health impact of all this dark matter slamming your body, most likely hitting the oxygen and hydrogen atoms. There is no rash of unexplained diseases or deaths that might be explained by dark matter interactions. But, there are health conditions that we don't understand.
View this month's star chart.