I was recently diagnosed with fibromyalgia and went looking to my family for support. The first words out of my father's mouth "is that even a real thing? I thought that's just what doctors said you had when they couldn't figure it out our thought it was all in your head." I just sat there staring at him trying not to cry, I didn't even know how to respond. What do you say to people who think this way?
Unfortunately, that is the most common thing that people will say.
And, the thing is, it's actually true if you look at it. It is the thing that doctor's diagnose if they have tested for everything else that could be causing the symptoms you have and all those have come up negative. And it is also the thing they say when they think it is all in your head.
That doesn't mean it IS all in your head, though.
I advise that you calmly and in an unemotional manner direct your father and anyone else in your life who tries to pooh-pooh it, to articles such as suggested by Jemima, and explain to them that fibro is one of several syndromes or conditions that you not always visible on the outside, but that are nevertheless very real.
As Jemima says, you also do not have to stick around if someone is unreceptive to learning about this, and you don't need to allow any unsupportive people to be in your life making things worse. If you can express this to your father in a very calm way (emotion will only makes it worse, and make him more likely to think it's in your head!), he may be willing to let you explain it to him or to investigate what it is.
He may have said that just out of ignorance, and not out of a lack of desire to be there for you. Ignorance is not a bad thing...it is always curable by gaining knowledge. If your father is willing to learn, this will change.