According to modern cosmology, the particles that constitute ordinary matter (protons, neutrons, and electrons) formed when the universe cooled after the Big Bang. These particles then came together to form hydrogen atoms, helium atoms, and small amounts of the next heavier element in the periodic table, lithium.
All the other elements in the universe were formed inside stars like the Sun and inside exploding stars known as supernovas. Through the addition of neutrons to lighter elements, nuclear reactions produced heavier elements. Supernovas dispersed these elements into interstellar space. Mixed with the hydrogen, helium, and lithium from the Big Bang, these elements formed our solar system.
Some atoms are radioactive, meaning that they naturally decay into other radioactive and nonradioactive atoms by emitting subatomic particles and energy. Each radioactive nuclide has a characteristic half-life, which is the amount of time it takes for half of the atoms in a sample to decay. Radioactive atoms therefore act as internal clocks for materials. By comparing the amount of a radioactive element in a material to the amount of its decay product, researchers can determine when the material formed. These measurements have yielded ages for the Earth, the Moon, meteorites, and the solar system. All of these measurements indicate that these objects are billions of years old.
[Nuclide: An atom with a particular number of protons and neutrons in its nucleus. An element is defined by the number of protons in its nucleus. Nuclides that have the same number of protons but different numbers of neutrons are isotopes of that element.]
Some who oppose the teaching of evolution try to cast doubt on radiometric age measurements. Radiometric dating is the product of more than a century of ingenious research and represents one of the most well-substantiated achievements of modern science.