Encounter cards (formerly Challenge cards) are a category of cards that are used to decide encounters. They are played face down during the planning phase and revealed during the reveal phase. Each main player must play one encounter card per encounter; allies do not play encounter cards. Encounter cards form the bulk of the contents of the cosmic deck.
Most encounter cards are attack cards, which have numerical values on them; these are added to the ships on the player's side to determine their side's total in an encounter. The second common type of encounter cards are negotiate (formerly compromise) cards, which are used to make deals or receive compensation. Other types of encounter cards, such as morph, are much more rare, and their text explains how to use them.