Actually there is a method to this madness..
Hinduism is a religion/philosophy based on scientific theories that have been refined over the ages.
Rishis of the past were highly educated scientists who gave their theories about life and universe.
There are two basic theories
1)
Big Bang theory: The sound of Aum represents God (energy). Even though it is written/denoted with the symbol, it is the energy which is the key.
It represents the energy that caused the Big Bang (Hiraṇyagarbha - 'golden womb') and thereby causing the Universes to come alive, It represents that energy which is making the universes to expand and it is that energy that would make the universes to collapse one day completing the full cycle. Note that Hindus believe in circular time calendar (Yugas).
When one chants AUM (sometimes written as OM)
a) A - First the closed lips open (representing the big bang) - This is represented by Brahma (The Creator)
b) U - Then one continues the sound (representing the expanding universes) - This is represented by Vishnu (The Sustainer)
c) M - This is when the lips close back (representing the collapsing of the universes) - This is represented by Shiva (The Destroyer)
The trinity of Brahma-Vishnu-Shiva are together represented as Adhi Shakti (literally meaning the first energy but representing all forms of energies in ensuring that this cycle continues)
Many people think that Hindus believe and worship multiple gods where as the truth is that Hindus worship the energy (in various forms) that keeps this cycle of universe running.
Energy can neither be created nor destroyed but transforms from one form to another is the underlying concept on which Hinduism is based on. Since Energy manifests in different forms and is omnipresent, you would see Hindus worshipping various forms of energy and life (which is a form of energy) from nature to objects of the universe to plants to animals. This also gave birth to the concept of "Aham Brahmasmi" , literally meaning I am god myself ,as every living creature has life and energy which is the manifestation of God.
2)
Steady State theory: Buddhism was based on this. Basically it said there was no Big Bang and Universe existed, exists and will continue to exit AS-IS. Hence, there is no energy or god. Hence, Buddhism was an atheistic theory. But later, Adi Shankaraacharya successfully convinced that that why their theory was false. Buddhist monks got convinced by the arguments put forwarded by Adi Shankaraacharya which resulted in Buddhists converting back to Hindus in India.
@Sarge When your Hindu friend was referring to Bhagawan, He was referring to this supreme energy