"at one with the universe."

Generally, the phrase “being one with the Universe” or “feeling one with the Universe” implies a sense of identity, resonance, compatibility with the goodness of the cosmic forces, the Universe. It is an empathy with the Universe and its beings.

Philosophically, the concept of one with the universe can also be understood in the context of a "Royal union" or Raja Yoga, as propounded by Patanjali in the Yoga Sutras. It is the balance of all the physical, mental, moral, and spiritual facilities to be able to attain one's higher self, feel love for all beings and hence be receptive to divine forces.

Verses 2:4-5 from the Ashtavakra Gita  are translated as:

As a wave,

Seething and foaming,

Is only water

So all creation,

Streaming out of the Self,

Is only the Self.

Consider a piece of cloth.

It is only threads!

So all creation,

When you look closely,

Is only the Self.