Unspace is a bit of dimensional nothingness, a non-dimension that is crossed by the Trans-Time Dimensional Portal.
Marvel Comics continuity
Megatron had conspired to destroy the Ark with a batch of explosives planted by Blackjack. However, Cloudburst and Ratchet used the Trans-Time Dimensional Portal to transport the explosives into Megatron's headquarters on Cybertron, mere seconds before they were set to explode. Megatron tried to escape through the still-active portal, but Ratchet tackled him. The explosion sent them both flying into the portal, but also disrupted its operation, shutting it down. Megatron and Ratchet, both in transit when the portal shut down, were left drifting in unspace, "a bit of nothingness between realities".
A while later, Nightbeat used an automated probe to search unspace for Ratchet's energy signature. He found him... but he was horribly fused to Megatron.