The notion of the universe being infinite is a subject of debate and speculation in cosmology. Our understanding of the universe is constantly evolving as new observations and theories emerge. Currently, scientists do not have definitive evidence to conclusively determine whether the universe is truly infinite or finite.
When scientists refer to the universe as infinite, it generally means that it is unbounded and has no spatial boundaries. This idea is based on extrapolations from observations and theoretical models. However, you are correct that our knowledge is limited, and our estimates are based on a relatively small amount of data compared to the vastness of the universe.
Scientists gather information through various observational techniques, such as studying the cosmic microwave background radiation, the distribution of galaxies, and the measurement of the universe's curvature. These data provide valuable insights into the large-scale structure of the universe. However, due to the limitations of our observational capabilities and the vastness of the cosmos, our understanding is inevitably incomplete.
It is important to note that scientific theories and models are continuously refined as new data becomes available. As technology advances and observational techniques improve, our understanding of the universe's size and structure may become more precise. The possibility of a finite universe or a universe with complex topology that gives the appearance of infinity is also being explored.
In summary, the idea of the universe being infinite is a current hypothesis based on available data and theoretical models. It is subject to ongoing investigation and revision as our understanding evolves.