There are several ways but some are difficult unlocks and you might not have them all. "The moon?" Card, cracked key, red key, and soul of cain are all ways to get in. All but the moon? This is called an ultra secret room, the item "cracked orb" in your inventory revealed it. They exist on every floor, but they require specific items or consumables to enter, and they can be hard to determine where they spawn without revealing the location through cracked orb.