Scientific reasons given for not believing in God
Science is based on the observation that the universe is governed by natural laws that can be tested and replicated through experiment and is used as a reliable and rational basis for prediction and engineering. Like a scientist, a scientific skeptic aims to decide claims based on verifiability and falsifiability rather than accepting claims on faith or relying on unfalsifiable categories, by utilizing critical thinking. (Opposite of what is known as the true-believer syndrome.)
Most theistic religions teach that mankind and the universe were created by one or more deities and that this deity continues to act in the universe. Many people, including both atheists and theists, feel that this view is in conflict with both the discoveries of modern science (especially in cosmology, astronomy, biology and quantum physics) and also the fundamental principles of science; Science and theistic religions are mutually exclusive philosophies. Many believers in the validity of science, acknowledging this contradiction, do not believe the existence of a deity or deities actively involved in the universe.
Evolutionary science describes how complex life has developed through a process of mutation, adaptation and natural selection. It asserts that every species of life on this planet, past and present, are products of a stochastic process. Similarly modern humans have only existed for the last 0.0015% (approximately 100,000 years) of the age of the universe, the Earth's Sun is one star among billions in the Milky Way, which is a galaxy among billions of others. It is also now known that humans share 98% of our genetic code with chimpanzees, 90% with mice, 21% with roundworms, and fully 7% with the bacterium E. coli. This scientific perspective is quite different from that of most theistic religions, which give humans a unique and central status (anthropic principle). In the Abrahamic religions, for instance, humans are thought to be created 'in God's image' and to be of a qualitatively different order of life than the 'beasts of the Earth'.
Scientific progress has been offered as a means to disprove religious claims. Most religions that involve supernatural entities and forces are linked to unexplained physical phenomena. In Ancient Greece, for instance, Hades was the god of the dead, Helios the god of the sun, Zeus the god of thunder, and Poseidon the god of earthquakes and the sea. In the absence of any scientific theory that could explain a given phenomenon, people who sought an explanation attributed its cause to supernatural forces, an argument that has come to be known as God of the gaps. Throughout history, most of these phenomena have been explained through the scientific method and found to conform to natural laws.