Because most of our universe is matter, there’s a lot of it around. Even the air you breathe is made up of millions and millions and millions of atoms, so antimatter would come in contact with these atoms if you made it in air.
Therefore you have to make it in a vacuum chamber (which isn’t that hard), but you also have to stop it hitting the walls of the chamber (which are matter) so you have to use a very very strong magnetic field to hold it still, and that’s very very hard.
The reaction between anti-matter and matter is called ‘annihilation’ (which I think is an awesome word!)
I don’t understand it very well, but matter has a positive mass, and anti matter has a negative mass. When they come together they annihilate to form a photon, which has zero mass. In this, it’s a bit like chemical reactions that work to even out charge in ions – so Na+ and Cl- come together to make NaCl, which is neutral. Think of it in terms of mass, and that’s what the anti-matter and matter do…