Time:

Time may seem like an easy thing to comprehend, but in fact, physicist still do not know what it is. Time is easily seen, but is not easily understood. When time is mentioned in physics terms, it isn't the earth's rotation about it's axis and around the sun that is difficult to understand, it's the idea of it. In physics, time is understood as an arrow. It is easily understood that our universe is constantly expanding. When looking at a mortar or any sort of explosive on a ground, you notice that the dirt shoots away from the ground in all directions. Take away gravity, and the dirt would continue to shoot off in the direction they were traveling.

Now, picture this dirt as individual galaxies. The continuous spreading of this dirt is what makes up the arrow of time. The origin of the explosion would be the beginning of the time measurement. What some physicist believe is that the forces and the motion of these galaxies is what makes time exist. What is unknown is how this is possible.

The leading researcher of this topic is Dr. Sean Carroll at Caltech. A great interview with Dr. Carroll explaining a better concept of time can be found here.