If what we see now demands that a process is in progress then we must explain our rationalizations or admit to a cyclical phenomenon which we are only allowed to witness; it is this nasty beginning bit that we can't get past.
Indeed, Einstein first proposed the cosmological constant and then ditched it because he felt it was 'kluged.'
But recently, scientists have brought the factor back into the equations because it was discovered that the universe is accelerating. Originally, the term dark energy was used interchangeably with dark matter. But now, they are defined separately. Dark matter has positive gravity - black holes and massive neutrinos (if any) whereas dark energy has negative gravity and thus, causes acceleration.
AIP 479: Dark Energy and the Microwave Background