The concept of the singularity in the Big Bang refers to a point of infinite density and temperature, where our current understanding of physics breaks down. It does not imply the absence of matter but rather represents a state where the laws of physics as we know them cease to apply.
After the Big Bang, the universe rapidly expanded and cooled. As it expanded, matter and energy became less densely packed, allowing for the formation of particles such as protons, neutrons, and electrons. During this phase, known as the cosmic nucleosynthesis era, light elements like hydrogen and helium were produced.
As the universe continued to expand and cool further, matter started to clump together under the influence of gravity. These regions of slightly higher density eventually led to the formation of the first galaxies. The process of galaxy formation is complex and still not fully understood, but it involves the gravitational collapse and merging of smaller structures, the accretion of gas and dust, and the formation of stars within galaxies.
So, while the singularity itself may not contain matter as we understand it, the subsequent evolution of the universe allowed for the formation of matter, including the building blocks of galaxies. The study of galaxy formation and evolution is an active area of research in astrophysics and cosmology, and ongoing observations and simulations contribute to our understanding of this process.