Depends on what you consider "science". Science as in, making things exploding, won't give you any meaningful result.
Jokes aside, there's never a fullfilling explanation as for why some people feel they are in a wrong body. Considering the diversity of the transgender landscape in our days, you just can't point at a single, unitary explanation. It is believed that biological reasons, such as genetic factors or prenatal hormones, combined with early childhood experiences and those in the adolescense contribute to the development of a different gender mindset from the body they were born with. Transgender people experience their identity in very different ways and in different ages; some may have feelings and explore gender-nonconforming attitudes at a very early stages of their lives.
