Thus there is no near-contemporary source of Anglo-Saxon origin. The reason is obvious enough: the Germanic peoples were illiterate during their first two centuries in Britain. So their early fortunes can only be glimpsed through the hostile eyes of Britons, through the ill-informed eyes of foreigners, and by means of their own half-remembered traditions. Until the late sixth century, informed guesswork must make do for history.
In the last sentence, I don't understand what does the phrase 'must make do' means. Is it an unformal expression?