Based on this information, we can assume that the Hulk managed to somehow fall through one of these intergalactic passages, fell on Planet Sakaar, and was captured by Valkyrie.
Obviously the picture isn’t entirely clear, but we are starting to get a solid idea of what happens.
So how does this puzzle piece fit into the grand mystery?
Sakaar is a planet surrounded by cosmic gateways, and when people/things fall through those gateways, it seems they wind up getting picked up by “Scrappers” (Valkyrie is identified as Scrapper 142 by The Grandmaster).