The Universe is mostly dark matter. Dark matter, in fact, is all over the place, and there's five times as much of it out there as there is regular matter. We have no idea what dark matter is, but there's one thing that we are pretty sure of: some of this mystery stuff is smacking into you once every 10 seconds. You may now start to panic.
What makes dark matter "dark" is the fact that it neither absorbs nor emits electromagnetic radiation, meaning that we can't detect it with telescopes. We can't really detect it with much of anything at all, but we know it's got to be out there because otherwise there's just too much gravity to go 'round. Since dark matter does have gravity but doesn't have anything else, we can only assume that it's made up of weakly interacting but massive particles (aka WIMPs) of some sort.
While we've never actually seen a WIMP up close, it's probably something like a neutrino, except much much more massive. Like neutrinos, WIMPs don't have any charge, so they can pass straight through ordinary matter unless they happen to smack directly into the nucleus of an atom. WIMPs aren't nearly as common as neutrinos, but they're probably still common enough that within the last 10 seconds, one of them collided with the nucleus of an atom somewhere in your body.
New calculations made by Katherine Freese at the University of Michigan in Ann Arbor and Christopher Savage at Stockholm University in Sweden have come up with this one per 10 second number, which is rather a lot more than the between five and 30 times a year that common assumptions about dark matter had previously suggested.
Now, whether WIMP strikes are a bad thing is debatable, but generally, having high energy and/or massive particles smashing into your DNA usually isn't something to look forward to. On the upside (or maybe it's a downside), there isn't anything at all we can do about this, although technically, if you were to wear a tinfoil hat it would slightly decrease the rate at which WIMPs would strike your brain. Just a thought.