I do believe there’s something not yet explained (but which WILL be explained scientifically at some point in the future) that explains the existence of trans people.
We are already some way towards explaining it.
If you do not meet the "norms" of heterosexual society, by being attracted to the opposite sex - there is for some people a drive to "normalise" your position by taking on some of the behaviour patterns that are stereotypically associated with the opposite sex. This seems to be in extreme cases (or in extreme times e.g. when male homosexuality is proscribed to the point of being illegal, when female stereotypes are exaggerated) to lead to a wish to BE the opposite sex.
If you have had some type of trauma that causes you to have negative feelings towards your body you are more likely to experience gender dysphoria, body dysmorphia, eating disorders etc etc.
If you have an identity crisis due to some other reason (e.g. adoption, being raised as a boy like some children in some Asian countries whose parents had a long string of girls) then you are more likely to experience gender dysphoria.
If you have ASD you are more likely to experience gender dysphoria, possibly due to "black and white thinking" or to a lower likelihood of adhering to social norms.
If you have a sexual preference which means you are turned on by the thought of yourself as a member of the opposite sex, you are more likely to want to dress as or alter yourself to appear more like the opposite sex.
These are all explanations that rely on things we already know about human behaviour. I don't feel that currently we need any "additional", gender-dysphoria-specific explanations.