Donation records associated with Default Contact should be updated to an actual Contact.
For more information about how these get created in your system, see Default Contacts.
The Customer Portal Report Card gives an A grade when less than 2% of your Donations are associated with Default Contact compared to number of Contacts with Activity in the last year (or none). Activity is based on the Activity Log for the last 365 days.
The Assign Donor Tool is able to fix Donations, automatically create Household, and optionally create a Participant for a Donation assigned to Default Contact.