How are players in casual games matched?

In Rocket League there are three online modes: a competitive mode, a casual mode and an extras mode. Competitive matches are usually formed by a similar difficulty level; players at the same levels or mixture of levels can usually be found player against one another. In casual, however, there doesn't seem to be an sort of ranking system involved. Surprisingly, these matches are usually fairly balanced. There must be some sort of system in place to ensure fair casual game play.

How are players in casual games matched?

There is a hidden numerical ranking, called your MMR ("Match-Making Rating"). You can see its value in-game using Bakkesmod, or out-of-game using Rocket League Tracker.

You have a separate hidden MMR for each ranked mode, but a single hidden MMR for all unranked modes. The rank spread is significantly larger for casual MMR - for example, 2000 MMR would be around pro level for ranked standard, but in casual 2000 MMR is filled with people around low Grand Champ.

In my experience, casual games are a lot more lenient about MMR-spread in matchups. In ranked modes, your opponents will usually be ±100 MMR of you, but in casual it's not uncommon to see opponents ±400 MMR.

For more information, see this extremely detailed reddit post.