Reason
When looking at the past, the only possible outcome to events, is the outcome that happened. There are no "what if"s. There are no "could have"s. There is only was.
When looking at the future, the number of outcomes are limitless. There are no guarantees. There are no certainties.
The past does not move. The future does not move. The now, moves.
Future, past and now, are three states of the same continuum.
Without now, there is no matter.
Without now, there is no space.
Without now, there is no energy.
Mater is only what we bump into while floating around in space. Without time to travel from one point to the next, mater, space, the energy used to pass from one to the other, need not exist. There for, everything in the universe only exists for the moment that it is being observed. By the time you realize a moment has past, it is gone. There for, it is impossible to for the universe to actually exist.