For more than sixty years we have known that heavy elements such as gold and platinum are produced in the Universe through a sequence of nuclear reactions that include neutron captures and beta decays known as the r-process. The identification of the astrophysical place where this process occurs has only been possible recently thanks to the theoretical suggestion that the production of gold in the Universe is associated with an electromagnetic signal with a brightness equivalent to one thousand novas known as kilonova. This signal was first observed in August 2017 shortly after the detection of gravitational waves produced by the collision of two neutron stars in the NGC 4993 galaxy located 130 million light years. This combination of observations in gravitational and electromagnetic waves constitutes a unique possibility to study the properties of matter at high densities and the origin of heavy elements in the Universe.