You hear over and over again to write what you know. The logic behind this is that you are bound to write it in greater detail and with a keen eye for accuracy. It is sound advice, except when it isn’t. What I mean by this is that having knowledge of something is indeed helpful, but it can also lead you to get sloppy.
Just because you already understand something doesn’t mean you can leave out details that help someone else with no experience to understand it. Worse, human memory is fallible. We forget minor things and remember what we each saw a little differently from the person beside us. Things we perceive now aren’t the same as was the case even just a few days before. If you do know the topic intimately, do research anyway.
What I advocate is not to write what you know, but rather get to know new things. Write about whatever you want, but take the time to really get to know that topic. Are you writing about a man lost in a frozen wilderness and trying to survive? Try camping out in the coldest part of winter and using the techniques you are describing so you understand how difficult it really is. Going to do a space opera where the heroine of your story works on the ship’s engines? See if you can’t get some time in at a space agency such as NASA for at least a few days along with interviews. Sure they don’t have space-fold drives, but it is going to be better than just looking under the hood of your car and then applying theoretical physics.
My point is that while many great writers did write what they knew, they only knew it because they first went out and did it. Go out and do things, learn for the sake of learning and never stop doing so. Follow the whims of interest. Feed your mind and it will be able to offer up any number of ideas at any time. Absorb information and apply it to your writing. Never stop learning. Never stop growing. Don’t write what you know, know what you want to write and learn it.