I think I would suggest modifiying that last question to read "Who's missing from the stories you have read?" What struck me as I got older (and what drives my mother up the wall as respected scholar specialising on Nordic history and languages) is the discrepancy between the way Vikings were portrayed in a lot of western Euopean history books and the way in which Swedish history books presented them. There was always this image of Vikings as a bunch of violent male sea-borne thieves, killers and rapists. Yet in reality (and in Swedish history books) their society was far more complex and advanced than that, with a focus on trade and complex political and social structures where women played a very important role."What is true."
As a medievalist and a student of language and literature, I'm pretty familiar with this idea, and I understand where it comes from. It's easy to imagine that a world lit only by fire would have pretty hard and fast delineations between disparate populations and ethnic groups. To each their own and all that.
That's not what actually happened.
We get the concept from much more recent scholars who didn't like what the things they were finding said about their perceptions of their world and the worlds that had gone before, so they edited it until it matched what they wanted to see. Entire populations were erased. Women leaders of every stripe, 'foreign' influences on learning, politics, religion, and philosophy were literally cut page by page from the record. So much is missing that only in recent years have we begun to understand the extent of the damage done.
One thing remains true, as it has been since we stood up and peered over the grasslands and feared the darkness: populations that trade, that migrate, that emerge from their caves at all, will mingle. The Vikings traveled the world. We are finding now that they weren't the people we thought they were. This pattern repeats over and over, and is reflected in our very DNA. What is true? It is more complicated than we ever thought possible, and populations were far more diverse and culturally rich than we give them credit for. The shadows and ghosts of that truth are still there in the record, in the blank spaces left behind. It's there in the question, "Who's missing from the stories?"
That experience always makes me wonder if there is something that I've missed in the research I do before I write, something that I'm not aware of which might have an impact on my characterisation or my setting. But maybe being aware of that risk is good enough as a start?