|Specifics: picture of Prime in infraspace from Escalation #6|
Not much is known of infraspace. A Transformer can seemingly exist there without a body, or perhaps it is where a Transformers' mind or spark exist when their bodies have been destroyed.
IDW comics continuity
Optimus Prime had received a severe beat-down from an Ore-13-powered Megatron, and was on the verge of terminal shutdown. In a desperate ploy, he transferred his higher functions into his trailer section, giving the appearance of lifelessness.
During the transfer, Prime's consciousness drifted through infraspace. While there, he felt a sinister presence that seemed to recognize and watch him.
The connection between infraspace and the Dead Universe, if any, has yet to be revealed.