The concept of the shape of the universe can be a bit complex and counterintuitive. When scientists refer to the universe being "flat," they are actually describing the geometry of space on a large scale rather than the shape of the universe itself. This concept comes from the study of cosmology and is based on observations and mathematical models.
According to current scientific understanding, the universe appears to be very close to flat on a large scale. This means that if you were to draw a triangle on an extremely large scale, the angles of the triangle would add up to 180 degrees, just like in Euclidean geometry.
The idea that the universe is expanding in all directions from every point does not necessarily imply a spherical shape. The expansion of the universe is more like the stretching of space itself, rather than the expansion of matter into some preexisting space. It is important to note that when scientists say the universe is expanding, they mean that the distances between galaxies and clusters of galaxies are increasing over time.
The concept of a spherical universe arises from the analogy of an inflating balloon, where the surface of the balloon represents the universe. As the balloon is inflated, any two points on its surface move away from each other, similar to the expansion of the universe. However, this analogy is not meant to represent the actual shape of the universe, but rather to explain the concept of expansion.
The flatness of the universe is supported by various observations, including measurements of the cosmic microwave background radiation, large-scale galaxy distributions, and studies of the geometry of space on a cosmological scale. These observations suggest that the universe is very close to flat, although small deviations from perfect flatness are still possible.
It's important to remember that our understanding of the universe's shape and geometry is based on current observations and theoretical models, and further research and observations may refine or expand our understanding in the future.