Final answer:
The universe is dominated by matter, evidenced by the lack of substantial annihilation radiation. Antimatter, rarely observed, is usually quickly annihilated by matter. Small asymmetry in fundamental forces in the early universe led to a matter-dominated universe, with ongoing research shedding light on the matter-antimatter asymmetry.
Step-by-step explanation:
While the universe we observe is overwhelmingly composed of matter, the concept of antimatter coexisting is a well-established aspect of particle physics. Theoretical and experimental evidence points to the existence of antimatter, such as in particle collisions and ß+ decays, although in very limited amounts compared to matter. Annihilation would occur should matter and antimatter meet, as described through Einstein's equation E = mc², where their mass would convert to energy, typically generating photons. This annihilation process leaves us with a universe that appears to be almost pure matter. This observation is backed by the lack of substantial annihilation radiation, such as the 0.511-MeV gamma rays that are expected from the mutual destruction of electrons and positrons, which would be abundant if antimatter was present in large quantities. However, a slight asymmetry in fundamental forces in the early universe caused slightly more matter to be produced, which after annihilation of matter-antimatter pairs, has resulted in the current matter-dominant universe.
Beyond direct observation, high-energy physics experiments have been able to create antimatter in controlled environments, such as antiprotons and antineutrons in accelerator experiments since 1956, and even an anti-hydrogen atom at CERN in 1995. These contribute to our understanding of how matter and antimatter interaction, though they are not evidence of significant antimatter presence in the universe. The ALPHA experiment at CERN and other studies like those referenced by Fermilab, continue to contribute to our understanding of how our matter-dominated universe came to be.