Antimatter

One way of viewing antimatter is as matter going backwards
in time. However, that really only works on a microscopic
scale.
Macroscopically antimatter is expected to behave exactly the
same as matter (gravity works the same, and the mass of a piece
of antimatter is always exactly the same as the mass of its
corresponding piece of matter) except that electrical things will
all be backwards, since antimatter has opposite electrical charges
from matter.