Why is the antimatter resting? 'Tis lazy?
Again I’ll proceed from the perhaps erroneous assumption that this question is at least partway serious. You must not read “inertia,” “inertial” and so on as referring to “a state of rest.” (According to which frame of reference is this “state of rest” to be judged anyway?) Rather, read “inertia,” etc. as indicating the
resistance to a change in the state of (uniform) motion a ponderable mass offers, as per Newton’s second law.
'Luthon64