According to Osho, kundalini is psychic—real yet nonmaterial. The actual phenomenon is an inner, forceful upsurge felt as a rising or penetration; ‘serpent,’ ‘flowering,’ ‘jet,’ etc., are symbols the mind supplies to understand and communicate it. Materiality is only one dimension of reality, so do not dismiss the psychic as unreal; attend to the experience beyond its metaphors.
Kundalini is a real inner energy you feel, not a physical thing, and we use images like snakes or rockets just to describe it.