If you just moved in, the previous tenant took their contract with them when moving out. If you don't take any further action, you will receive a letter from the local default provider (Grundversorger). There is nothing wrong with that, however electricity rates are often cheaper with competition especially with price guarantees & bonuses. Many don't bother switching supplier, costing them hundred of euros. When signing up for a new contract, your new provider will notify the previous one that the contract has changed hands.