*Takes a deep breath*
The visible universe is made up of 4D particles, of whom we can only perceive a small, 3D cross-section. As with a 2D cross-section of a 3D object, the cross-section is only a small part of the whole, and likewise, the 3D cross-sections of the 4D particles that make up the universe are much smaller than the actual size of the 4D particles themselves. This is what Dark Matter is. Dark Matter is nothing more than the rest of the 4D particle that we cannot perceive. We can observe gravity, because it is a 1D effect, causing two particles, in the absence of other factors, such as inertia, to move toward each other in a perfectly straight line.
Time is a 5D fluid that is gathered by the gravity thrown off by the 4D particles, but has no gravity of it's own. The 4D particles are suspended within the 5D sea of Time-Fluid, and interact with each other, creating complex patterns, not entirely dissimilar to that of dust on the surface of a pond. These patterns make up everything that we perceive as matter and the universe in our 3D existence. As the amount of 4D particles increase, so does the amount of gravity they throw off, causing the 5D Time-Fluid to become more dense, and causing the pattern of particles suspended within it to move more slowly, much like the slowing of dirt suspended in cooling molasses. We perceive this as the slowing of time in environments of increased gravity.
This means that time travel is completely impossible, because time is merely the 5D fluid that the 4D particles that make up our universe are suspended in. Patterns continually shift within the Time-Fluid and are then gone. The Past no longer exists in any way, and likewise, the future does not exist. Time travel is impossible because time, as we perceive it, is merely the speed of shifting patterns within the 5D Time-Fluid.
*resumes normal breathing*