The Dimension Gates appear to be the means by which the Alien craft travel from their home world. Our craft cannot travel through these gates without being annihilated by an anti-matter implosion. We must disable a UFO and recover its control systems, propulsion systems and power sources for research.
The Dimensional Gates are used to go to and from the Alien Dimension.
No standard human vehicle can enter a dimensional gate without being annihilated by anti-matter.
Once a number of captured alien craft and their control, propulsion and energy systems are researched, it is possible to build a Dimension Probe that can enter a dimensional gate and begin the exploration of the Alien Dimension.