Well, according to what I have gleaned from the books published by Stephen Hawking and John Gribbin, yes, it does exist. However, it is not the ever-flowing steady unidirectional stream into the future that we often think of it as. Einstein's Theory of General Relativity (or maybe Special Relativity, I can't remember) basically states that space and time are inseperable. Various studies of the "twin paradox" have brought this point home. Time is relative to the observer. Therefore, speed determines how time is perceived. To us, time usually drifts by in 60 minute hours, 24 hour days, etc. However, if someone is traveling at high velocity (small speed variations are negligible) time for them passes much slower according to our perception on the ground. The idea of time gets even more outrageous when one considers that time itself stops within the boundary of a black hole because it would take an infinite amount of time between clock ticks for anything to escape.