Time.

[Darwinian]

There is no such thing as a future, it hasn't happened yet, and at least the past happened once. All we have left is the fleeting moment of the present.

If the future didn't exist then how is it that matter/antimatter particles can be observed?

To come into existence they borrow energy from the future to create a particle of matter and antimatter which annihilate one another to repay the "energy debt" this happens all the time and is observable in empty regions of space. So long as the energy is payed in a time frame short enough, which is determined by Heisenburg's uncertainty principle then this is acceptable in the realm of quantum physics.