Neutrinos are tiny, almost weightless particles that only interact via gravity and nuclear
decay.
Because they don't interact electromagnetically, that is, with light, they literally can't
be seen!
In fact, detecting a neutrino is kind of like trying to catch a bullet with a butterfly
net – a beam of neutrinos will travel through lead for two years before it stops.
(in comparison, radiation from nuclear fallout can be blocked by about ten cm of lead)
So how do you detect a neutrino?
One common way is to fill a big tank with water: we know light slows down through water,
and if a neutrino with enough energy happens to knock into an electron, the electron will
zip through the water faster than the light does!
When this happens, the electron gives off a weak glow called Cherenkov Radiation - it's
kind of like a sonic boom for light, and it allows us to detect the neutrino.
The biggest neutrino detector in the world is a balloon over the south pole that actually
uses the whole antarctic ice sheet as its tank of water!
Neutrinos also tell us that the universe is not the same as its mirror image.
If you switch left with right, clockwise with counterclockwise, almost all of physics, like
gravity, electromagnetism, and the strong nuclear force, is unchanged.
However, the weird thing about neutrinos is that in physics terms, they're all left-handed
- their mirror image doesn't exist!
So neutrinos are the vampires of physics.