All matter we know has a positive mass and is attracted to other matter due to gravity – like an apple falling to Earth. Matter with a negative mass would push other matter away from it.

A politician, historian and academic answer kids' questions on Black Lives Matter.

What is exotic matter, and could we use it to make wormholes? – Julia, aged 14, London Matter is “stuff”. It is anything that is made up of particles that take up space. Everything we can ...