Default list on credit profile with no proof

Is it legal practise to place a defaulter listing on somebody if you do not have valid proof of money owed, proof of you having any dealings with a company, proof of you owing a business anything, no contracts, no agreements, no payment dates or any specified payment amounts, zero paperwork?

Is it legal practise by Transunion to place a defaulter listing on a credit profile without any valid or guaranteed evidence proving an individual owes a business money or do you just take a debt collectors word or an agent that depends on debt collectors for income to place defaults on credit profiles?

Emails and any type of contact with Transunion is non existent.