Powers and other effects often deal specific types of damage. For instance, a red dragon’s breath deals fire damage, a scorpion’s sting deals poison damage, a mind flayer’s telepathic blast deals psychic damage, and a wraith’s touch deals necrotic damage.
If a power doesn’t specify a damage type, the damage has no type. Most weapon attack powers deal damage that has no type. It is simply physical damage.
Published in Rules Compendium, page(s) 223, Monster Vault: Threats to the Nentir Vale, page(s) 122.