I have come across many symbols like ॐ , Swastika , Yantra and Lord Shiva's Trident that are generally associated with Hinduism.
Just like the cross represents Christianity or crescent represents Islam, is there any universal symbol for Hinduism?
Aum ॐ is considered as the Bramha Naad the first naad or sound made after the creation this was considered to be made out of the Big Bang Explosion (HiranyaGarbha(golden Cosmic Egg) Expanding).Scientifically you can think it as the CMB (Cosmic Microwave Background radation) left as traces after the Incident of Big Bang. As Aum is complete in itself and the first naad after creation it is also considered as Mantra Raaj (king of all mantras) as Mantras are the made out of naad (vibes or energy waves) , ॐ is Sampurna Naad (whole naad). That's why ॐ is not only a symbol for hinduism but for everything in this universe , it contains the whole universe in itself ,it is the universe itself not just a
universal symbol ,and talking about Sanatan (hindu) Dharma it is the one which talks only about supreme truth, so we have taken veda vaani (word of vedas) to make ॐ our universal symbol(vedas depict aum as the Bramha Naad).