Our current understanding of the universe is based on scientific observations, theories, and models. While it is difficult to be absolutely certain about the nature of the universe, the current evidence and understanding suggest the following regarding your questions:
Other Galaxies: We know that there are other galaxies beyond our own through astronomical observations. When astronomers study the night sky, they observe numerous galaxies, each containing billions or even trillions of stars. The concept of galaxies beyond our own was established through the work of astronomers like Edwin Hubble in the 1920s. They observed that distant objects were receding from us, indicating an expanding universe. Additionally, observations of the cosmic microwave background radiation, which is the afterglow of the Big Bang, also support the existence of other galaxies in the early universe.
Multiverse Theory: The concept of a multiverse, where our universe is just one of many, is a theoretical idea proposed by some physicists and cosmologists. It suggests that there could be multiple universes with their own physical laws and properties, possibly coexisting alongside ours. However, it is important to note that the multiverse theory is currently speculative and lacks empirical evidence.
Scientists explore the multiverse theory through theoretical physics, such as string theory and inflationary cosmology. These theories suggest the possibility of a multiverse but have not yet provided definitive proof. Testing and verifying the existence of a multiverse is extremely challenging since it goes beyond our current observational capabilities.
In summary, our knowledge is based on the current evidence and understanding, and it suggests the existence of other galaxies but does not provide conclusive proof of a multiverse. Scientific exploration and advancements in technology continue to push the boundaries of our understanding, and future discoveries may shed more light on these intriguing questions.