If a cognizable offence appears to have been committed, as per the complaint, police is bound to register the FIR. At that stage, evidence is not insisted upon. Thus, FIR can be filed without evidence being available at that stage.
It goes without saying that at the time of registration of FIR, in mot of the cases no evidence is available.
It is during the investigation of the FIR that the police collects evidence / material to examine that the offence actually has been committed, and if so, who has committed the offence and what is the evidence to support it.