Non-binary, more modern alternative to "Dear Sir/Madam"
I would suggest something along the lines of:


  Dear Admissions Committee,


or


  Dear Department of (STEM),


etc. You can usually find what a particular department/program/etc is called from their website. Avoid addressing the "Department of Chemical Engineering" as the "Engineering Department"; it would be most polite to use their own phrasing because it shows you've paid some attention and aren't sending out a copy-pasted letter to everyone.

For another example, I just randomly looked at a particular department, the "Department of Chemical Engineering and Biotechnology" at the University of Cambridge; for their "MBE"  program they have a "contact" page that says to "please send an email to the MBE Office": in that case I'd simply address the email:


  Dear MBE Office,


For any of the above, you can also omit the "Dear".

In a comment, @DanRomik suggests modifying the salutations to refer to particular groups of people (like "committee" does) rather than abstract departments: examples include: “Dear MBE program administrators”, “Dear MBE office team”. I think that's perfectly fine, too, if you find that more logical.