Before indulgence in Brahman Hinduism we must clearly know which God we are talking about, the manifest God or man gods or God Almighty himself.
Brahman an entity in Hinduism can be understood from spirituality point of view. Spiritual scriptures like Bhagavad Gita and Upanishads say Brahman is the ultimate creator of entire cosmos (cosmos is called Brahmaand in Hinduism).
It is Brahman Hinduism that is also called Parmatman, Sanatana Purusha, Ishwar, Bhagwan etc.
The present cosmos came into being when Brahman God Almighty exploded self with a big bang.
Brahman is the singularity in the beginning of cosmos which scientists world over of often discussed about.
Hinduism scriptures say Brahman exists from times immemorial. There was no time in the entire history of mankind, the entire cosmos that Brahman did not exist. Even before formation of cosmos Brahman was there and even after. Even after Pralaya occurs (dissolution of entire cosmos), Brahman would continue to exist!
Adi Shankaracharya details definition of Brahman very aptly. According to philosophy of Advaita Vedanta propagated by Adi Shankaracharya, other than Brahman nothing existed in cosmos. This is also true as at the time of big bang nothing other than Brahman existed.
Per se Indian spirituality everything visible in this world does not seem to exist. Even quantum theory confirms this. Broken down to basic building block of cosmos every single element in this world including human beings was gaseous formation, clusters of atoms and molecules nothing more. Only when perceived from senses point of view things appeared solid, in reality all was gaseous.
Even doctrine of Maya propagated by Adi Shankaracharya further confirms existence of Brahman and only Brahman in entire cosmos. As per doctrine of Maya the present cosmos does not seem to have a boundary. If it does have, then what beyond!
If the entire cosmos is without a boundary, it could exist only in thoughts of God Almighty similar to existence of an image in a mirror.
To indulge in Brahman, reaching stage of self realization, god realization indulgence in spirituality as detailed in Bhagavad Gita and Upanishads (independent spiritual treatises) is necessitated.
Brahman is that cosmic spirit behind every living form that was the cause of its existence, the very fact of life, Prana!