In modern cosmology and astrophysics, the presence of dark matter in the universe is today a central tenet and its existence in galaxies is well established. And when dark matter first was established in 1970s scientist used a variety of techniques which included the measurement of the rotation speed of gas and stars, which provided scientists a way to effectively ‘weigh’ the host galaxy and determine its total mass. These measurements showed that the visible matter only accounts for a fraction of the total weight, the predominant part is delivered by dark matter in the outer parts of the Milky Way is well established.
Using this technique to measure our own Galaxy has shown the diameter to be about 100,000 lightyears. Our Solar System is located at a distance of about 26,000 light years from the center. Coming closer to the center of our galaxy it becomes increasingly difficult to measure the rotation of gas and stars with the needed precision.
However Earth is not anywhere near the center — NASA says we’re roughly 165 quadrillion miles from the galaxy’s black hole, for example — which demonstrates just how big our galaxy is. So how big is it, and how does it measure up with other neighborhood residents?
NASA estimated the galaxy at 100,000 light-years across by using one light year is about 9.5 x 10 km, so the diameter of the Milky Way galaxy is about 9.5 x 10 km in diameter. The thickness of the galaxy ranges depending on how close you are to the center, but it’s tens of thousands of light-years across. The Milky Way is a barred spiral galaxy that and is estimated to contain 100–400 billion stars, although this number may be as high as one trillion. There are likely at least 100 billion planets in the Milky Way. The Solar System is located within the disk, about 27,000 light-years from the Galactic Center, on the inner edge of one of the spiral-shaped concentrations of gas and dust called the Orion Arm. The stars in the inner ≈10,000 light-years form a bulge and one or more bars that radiate from the bulge. The very center is marked by an intense radio source, named Sagittarius A*, which is likely to be a supermassive black hole.
Our galaxy is part of a collection known as the Local Group. Because some of these galaxies are prominent in our sky, the names tend to be familiar. The Milky Way is on a collision course with the most massive member of the group, called M31 or the Andromeda Galaxy. The Milky Way is the second-largest member, with M33 (the Triangulum Galaxy) the third-largest, NASA says. Andromeda appears much brighter in the night sky due to its size and relatively closer distance. There are about 30 members of this group.