A complete answer to this question assumes that the individual posing it is interested in a purely scientific explanation for the origins of the universe and is not inquiring about those origins from a Biblical or "creationist" perspective.
When discussing the origins of the universe, it is important to recognize the scientific contributions of Albert Einstein, whose theory of relativity was immeasurably important in the study of astronomy, and to the scientists and engineers who designed, developed, built, and launched into space the Hubble Space Telescope, which has enabled astronomers to see farther into the universe than previously thought imaginable.
The most widely currently accepted theory of the origins of the universe date its birth at about 13.7 billion years ago, when enormously hot gases condensced into a concentration so hot and containing so much energy that in an instant -- less than a human second of time -- it suddenly and violently expanded outward in what is commonly referred to as "the Big Bang." The resulting explosion of mass contained the substances of what became stars, planets, asteroids, comets, and, of particular importance for the study of the universe, "dark matter."
Over the next 15 billion years, that matter has continued to expand outward. The Hubble Space Telescope has enabled astronomers to "see" far into the past -- closer to what the universe looked like at its beginning than ever before. If funding is available, the James Webb Space Telescope will eventually replace the Hubble, and scientists hope that it will enable them to view even beyond the impressive limits of Hubble, and make visible for the first time the universe at its beginnings.
To understand the origins of the universe, one must think in terms of the speed of light -- in effect, how far light travels in a single year. Astronomers speak in terms of millions and even billions of "light years" when studying the distances between points in the universe, for example, the distance between the Sun and other stars in the Milky Way galaxy, or between the Milky Way galaxy and the Andromeda galaxy. These calculations help astronomers to make educated determinations of the age of the universe, and of our own solar system. By using radar, telescopes, and mathmatical calculations, they measure how distance in light years, and then extrapolate data to determine the age of the universe. The most accurate current estimates place the age of the universe at the aformentioned 13.7 billion years.
While the "Big Bang" theory of the origins of the universe has enjoyed much support, new data continues to be collected from telescopes and NASA-launched space probes. As more and more data is collected, even the Big Bang theory has started to have its critics