Ancient Wisdom and Quantum Physics both reveal the Oneness of the Universe ?

Is everything really just One ?

I have been reading some Advaita Vedanta, the Isha Upanishad and some other Non Dualist Philosophies.

The more I read, the more I am starting to believe that everything is One.

Eastern and Western Mystics teach the same thing. The Ultimate Oneness of the Universe and Everything.

Advaita Vedanta (Sanskrit: अद्वैत वेदान्त, IAST: Advaita Vedānta, literally, "not-duality") is a school of Hindu philosophy, and originally known as Puruṣavāda, is a classic system of spiritual realization in Hindu tradition.
The term Advaita refers to its idea that the true self is the same as the highest metaphysical reality of Universe, Brahman.

Source: https://en.wikipedia.org/wiki/Advaita_Vedanta

This is sometimes called Monism.

Monism attributes oneness or singleness (Greek: μόνος) to a concept e.g., existence. Various kinds of monism can be distinguished:

Existence monism posits that, strictly speaking, there exists only a single thing, the Universe, which can only be artificially and arbitrarily divided into many things.

Substance monism asserts that a variety of existing things can be explained in terms of a single reality or substance. Substance monism posits that only one kind of stuff exists, although many things may be made up of this stuff, e.g., matter or mind.

Source: https://en.wikipedia.org/wiki/Monism