So, this 'Domo' can be used together with 'Arigato. ' as in 'domo arigato' meaning “Thank you so much.” Just saying 'domo' would be less polite than 'Arigato' since it's the short version of 'Domo arigato. ' People use 'domo' rather than 'arigato' when they consider 'arigato' is a little bit formal in a situation.