It is not antimatter, but matter with negative mass/energy. There is a big difference: antimatter is known to exist and we can even produce it (although it is very difficult/expensive to produce and store). Matter with negative mass is not known to exist at all. Some people (non-physicists) claim that they can use the casimir effect for a proof of concept. Last time I looked at this, mainstream physicists consider this nonsense. This does not necessarily mean that it can't be done, but we should be clear that it is not supported by mainstream physics. Also, it is important to note that even in the best case this is only useful to detect if spacetime can be warped in such a way. It is very clear that this particular method, even if it works, cannot be used for actual propulsion, so you still need to find actual negative mass/energy if you want to build an Alcubierre drive capable of propelling a ship. Another point, that is not usually brought up is that if you can actually build an Alcubierre drive (and assuming General Relativity is correct) then it immediately follows that you can also build a time machine. This does not necessarily mean that the Alcubierre drive is impossible but it is important to note and in fact if it were possible it would be far more significant than actual faster-than-light travel.