It may be bitul Torah if you learned science while you were supposed to be learning gemara. Just as it would be bitul Torah to shake lulav while you’re supposed to be learning gemara.
If you are not engaged in Talmud Torah, studying science is a kiyum of the mitzvah of ahavas haborei. You can come to love hashem by seeing just how amazing his world is. You can see the quantum-complexity of an electron cloud to the biological harmoniousness of the human body. This is a mitzah miduarrisah.