Previously, I have mentioned how the universe is made of CHOCOLATE.
It surrounds us and penetrates us and binds the galaxy together… no wait, that's the FORCE!
Anyway, scientists call this delicious substance DARK MATTER, because it appears – or rather DISappears – to be invisible. Yet it makes up FAR MORE of the Universe than bits we can see.
(In fact, 70% of the universe is dark ENERGY which is even MORE mysterious but if the sums add up right then a quarter of everything is dark matter; that's FIVE TIMES as much dark matter as light matter!)
So you can't see it but you CAN see the effect that it has. In fact, using the Hubble Space Telescope, you can photograph a bit of a GLOW around distant galaxies caused by the Dark Matter BENDING LIGHT around the edges!
Now, there's a BIT of a puzzle about what dark matter is really made of.
Some people think it might just be ORDINARY matter bundled up into super-heavy lumps that, for some reason we don't understand, don't heat up and radiate light.
('Cos that's what USUALLY happens when ordinary matter gets bundled up into super-heavy lumps… we call them STARS.)
These bundles of ordinary-but-lumpy stuff are referred to as Massive Astrophysical Compact Halo Objects or MACHOs. Yes, scientists are like that; live with it.
Others think that there are a whole new farm of sub-atomic particles that carry MASS (so can affect gravity) but none of the other fundamental forces (so they DON'T interact with ordinary matter in any other way). These are called weakly interacting (because they don't really interact) massive (because they have mass, not because they are enormous) particles. Or WIMPs. Yes, I know; never mind.
Detecting the presence of actual invisible particles, though, would mean that there actually ARE invisible particles, and would settle that argument.
But how do you detect them if you can't see them.
Well, you CAN, if you are very, very careful, detect a teeny-tiny bit of a REBOUND in a bit of light matter, something that you CAN see, if a dark matter particle happens to collide with it!
And that is just what scientists at the bottom of a very deep mine in Americaland claim to have done.
Alternatively, it's students from the FUTURE messing about with the Large Hairdo Collider!