The concept of the Big Bang singularity refers to the initial state of the universe at the very beginning of its expansion. It is a theoretical point of infinite density and temperature where the laws of physics, as we currently understand them, break down. The singularity does not refer to the entire universe as we observe it today, but rather to a point of extreme conditions in the past.
When scientists describe the singularity as having infinite mass and density, it is based on mathematical models that attempt to explain the universe's evolution. However, it is important to note that these models break down at the singularity, and our current understanding of physics cannot precisely describe what happened at that moment.
As the universe began to expand and cool down, it entered a phase where particles and energy could form. The fundamental particles, such as protons, neutrons, and electrons, eventually combined to form atoms. The estimation of the number of atoms in the observable universe, approximately 10^80, is based on observations and calculations of the universe's current state.
While we can measure and estimate the number of atoms in the observable universe, it does not contradict the idea of the singularity at the beginning of the Big Bang. The singularity refers to a point of extreme conditions, where the known laws of physics cannot accurately describe what occurred. It is a concept that arises from our current understanding of the universe's evolution, and it represents the initial state of the expansion from which the universe emerged.