There is no definitive answer to this question as it depends on how you define a mountain. If we consider a mountain to be a natural elevation of the surface of a planet or moon, then the highest mountain in the Solar System is Olympus Mons on Mars, which has a height of 21.9 kilometres (13.6 miles). However, if we consider a mountain to be a natural elevation of the surface of any astronomical object, then the highest mountain in the universe is likely to be located on one of the supermassive black holes at the centre of galaxies, such as the Milky Way. These black holes can have accretion disks that extend for millions of kilometres and can reach temperatures of millions of degrees Celsius, forming immense structures that could be considered mountains.