You're asking one of the most important and complex questions in gender science: if being transgender is not due to a disorder of sex development (DSD), and not purely psychological, what is it? And yes — there is active research into biological (including physiological, neurological, genetic, and developmental) contributions to gender identity that do not fall under DSD.

1. Brain structure and neurobiology
There’s evidence from neuroimaging studies that the brains of transgender people sometimes show features more similar to their experienced gender than to their sex assigned at birth:
INAH-3 nucleus in the hypothalamus: Smaller in trans women than in cis men, more similar to cis women.
White matter microstructure and functional connectivity: In some studies, these align more closely with gender identity than assigned sex.
Caveats: Results are variable across individuals, and there’s debate about causality (innate vs shaped by experience or hormones).

2. Genetics and epigenetics
Some genetic and epigenetic factors might contribute to gender identity:
Twin studies show higher concordance for transgender identity among identical twins than fraternal twins — suggesting a heritable component, though not deterministic.
Studies have looked at genes related to:
Sex hormone receptors (e.g. androgen receptor gene variants)
Steroid metabolism
Brain development genes
No single “trans gene” exists — it’s likely a polygenic and multifactorial trait, like most complex human traits.

3. Prenatal hormone exposure
One well-supported hypothesis: gender identity may be shaped by hormone exposure in utero:
Brain sexual differentiation occurs at a different time from genital development in the fetus.
If there is atypical timing or sensitivity to androgens, this may lead to a brain that “develops male” or “female” differently from the body.
This model is not considered a DSD unless there is an identifiable disorder of hormone synthesis or action — which is rare.

4. Physics or fundamental biological models?
No, there’s no current research indicating that physics in the sense of particle physics or quantum theory directly causes gender identity differences. However:
Some speculative or fringe theories explore non-classical developmental biology, epigenetics, or chaotic systems modeling — these are not part of mainstream science.
There is growing interest in understanding developmental patterning and self-organization of sex/gender differentiation, which could someday involve deeper physical principles (e.g., morphogen gradients, cellular signaling cascades), but still within the realm of biology rather than physics as such.

5. Sociocultural and embodied cognition models
Although not biological, some active investigation focuses on how gender identity emerges from the interaction of body, brain, and social context. These models see identity as real and valid, even if not reducible to biology alone.