Death is the beginning

[Edwardo Piet]

No; it never ends there. Our bodies unite with the universe. It was proven scientifically.

What exactly does “unite with the universe” mean? And who proved it? What is the proof?

And how can we reunite with something we're permanently united with already?

We're already part of the universe. When we die, our body will still exist... and the matter that made up our mind will still exist... but the thing about how physics works is: Things will change form. And it does at least seem like consciousness dies along with the body... so presumably the energy of consciousness is still there but it seems that energy is no longer consciousness...

Well, no longer classic human consciousness at least!

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Death rhymes with bad breath.

I yam wut I yam.

And Lut* yiz wut 'e yiz.

*Kitkat. Had to say Lut, rhymes with wut.

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What exactly does “unite with the universe” mean? And who proved it? What is the proof?

We decompose when we die.
Scientifically, everything gets a piece of you, from insects to environmental factors, so the universe -with its components- eats you.

https://www.aftermath.com/content/human-decomposition

The universe eats you.

It would be much more correct to say: The universe eats part of itself. We are parts of the universe.

Lebiniz's windowless monads does spring to mind.

Gottfried Leibniz's theory of pre-established harmony (French: harmonie préétablie) is a philosophical theory about causation under which every "substance" affects only itself, but all the substances (both bodies and minds) in the world nevertheless seem to causally interact with each other because they have been programmed by God in advance to "harmonize" with each other. Leibniz's term for these substances was "monads" which he described in a popular work (Monadology §7) as "windowless".

An example:

An apple falls on Alice's head, apparently causing the experience of pain in her mind. In fact, the apple does not cause the pain—the pain is caused by some previous state of Alice's mind. If Alice then seems to shake her hand in anger, it is not actually her mind that causes this, but some previous state of her hand.
Leibniz's theory is best known as a solution to the mind-body problem of how mind can interact with the body. However, Leibniz also rejected the idea of physical bodies affecting each other, and explained all physical causation in this way.

Under pre-established harmony, the preprogramming of each mind must be extremely complex, since only it itself causes its own thoughts or movements, for as long as it exists. In order to appear to interact, each substance's "program" must contain a description of either the entire universe, or of how the object is to behave at all times, during all "interactions" which will appear to occur.

It can also be noted that if a mind behaves as a windowless monad, there is no need for any other object to exist in order to create that mind's sense perceptions, leading to a solipsistic universe consisting only of that mind. Leibniz seems to admit this in his Discourse on Metaphysics section 14. However, he claims that his Principle of Harmony, according to which God creates the best and most harmonious world possible, dictates that the perceptions (internal states) of each monad "expresses" the world in its entirety, and the world expressed by the monad actually exists. Although Leibniz says that each monad is "windowless," he also claims that it functions as a "mirror" of the entire created universe.

On occasion, Leibniz styled himself as "the author of the system of preestablished harmony"

I absolutely don't believe in any sort of god at all... but it's fun to think about.

Here's a basic summary of who Leibniz was if you have never heard of him:

Gottfried Wilhelm (von) Leibniz (/ˈlaɪbnɪts/; German: [ˈɡɔtfʁiːt ˈvɪlhɛlm fɔn ˈlaɪbnɪts] or [ˈlaɪpnɪts]; French: Godefroi Guillaume Leibnitz; 1 July 1646 [O.S. 21 June] – 14 November 1716) was a German polymath and philosopher who occupies a prominent place in the history of mathematics and the history of philosophy, having developed differential and integral calculus independently of Isaac Newton. Leibniz's notation has been widely used ever since it was published. It was only in the 20th century that his Law of Continuity and Transcendental Law of Homogeneity found mathematical implementation (by means of non-standard analysis). He became one of the most prolific inventors in the field of mechanical calculators. While working on adding automatic multiplication and division to Pascal's calculator, he was the first to describe a pinwheel calculator in 1685 and invented the Leibniz wheel, used in the arithmometer, the first mass-produced mechanical calculator. He also refined the binary number system, which is the foundation of virtually all digital computers.

In philosophy, Leibniz is most noted for his optimism, i.e. his conclusion that our Universe is, in a restricted sense, the best possible one that God could have created, an idea that was often lampooned by others such as Voltaire. Leibniz, along with René Descartes and Baruch Spinoza, was one of the three great 17th-century advocates of rationalism. The work of Leibniz anticipated modern logic and analytic philosophy, but his philosophy also looks back to the scholastic tradition, in which conclusions are produced by applying reason to first principles or prior definitions rather than to empirical evidence.

Leibniz made major contributions to physics and technology, and anticipated notions that surfaced much later in philosophy, probability theory, biology, medicine, geology, psychology, linguistics, and computer science. He wrote works on philosophy, politics, law, ethics, theology, history, and philology. Leibniz also contributed to the field of library science. While serving as overseer of the Wolfenbüttel library in Germany, he devised a cataloging system that would serve as a guide for many of Europe's largest libraries. Leibniz's contributions to this vast array of subjects were scattered in various learned journals, in tens of thousands of letters, and in unpublished manuscripts. He wrote in several languages, but primarily in Latin, French, and German. There is no complete gathering of the writings of Leibniz translated into English.