Overview from Wikipedia...
Eureka is Poe's last major work and his longest non-fiction work at nearly 40,000 words in length. The work has its origins in a lecture Poe presented on February 3, 1848 titled "On The Cosmography of the Universe" at the Society Library in New York. He had expected an audience of hundreds; only 60 attended and were confused by the topic. Poe had hoped the profits from the lecture would cover expenses for the production of his new journal The Stylus.
Eureka is Poe's attempt at explaining the universe, using his general proposition "Because Nothing was, therefore All Things are." In it, Poe discusses man's relationship to God and the universe or, as he offers at the beginning: "I design to speak of the Physical, Metaphysical and Mathematical – of the Material and Spiritual Universe: of its Essence, its Origin, its Creation, its Present Condition and its Destiny." In keeping with this design, Poe concludes "that space and duration are one" and that matter and spirit are made of the same essence. Poe suggests that people have a natural tendency to believe in themselves as infinite with nothing greater than their soul - such thoughts stem from man's residual feelings from when each shared an original identity with God. Ultimately individual consciousnesses will collapse back into a similar single mass, a "final ingathering" where the "myriads of individual Intelligences become blended." Likewise, Poe saw the universe itself as infinitely expanding and collapsing like a divine heartbeat which constantly rejuvenates itself, also implying a sort of deathlessness. In fact, because the soul is a part of this constant throbbing, after dying, all people, in essence, become God.
Online version of the Poem can be found at http://www.eapoe.org/works/editions/eurekac.htm