It's food made by Zess T. in Paper Mario: The Thousand-Year Door. It hasthe same properties as a Mushroom (heals five HP), except that it also makes the user Allergic, which means that she/he is immune to status effects, including Power Lift. It's made by mixing a Dried Bouquet with just about any other healing item in the game (except a Volt Shroom, Fire Flower, Syrup, Zess Dynamite, Courage Meal, Coconut Bomb, or a Trial Stew; even so, the resulting Mistake may be combined with another Dried Bouquet to make Space Food.) The most efficient way of cooking Space Food is by mixing a Dried Bouquet with a Dried Shroom; these only cost two Coins and can be bought very close to Zess T.'s house.
If Mario sleeps at the Inn in Fahr Outpost, a Space Food will be on the table when he wakes up. The Inn costs fifteen coins for the night, and the Space Food sells in the neighboring shop for ten coins, so in effect, the player can stay at the Inn for five coins.
Space Food also appears in Super Paper Mario, where it restores ten HP normally, but restores fifty HP in Outer Space.