Fourty-two years ago, it was shown that if the mass density distribution of an object is known, time-delay from gravitational lensing caused by that object can be used to determine the Hubble constant. Today, the Hubble constant is more accuratly determined through other means, but then the original quest can be turned upside-down, and time-delay plus the value of the Hubble constant can be used to accurately determine the (dark) matter distribution in the object. We will also look at other examples of how gravitational lensing can be used to detect dark matter in the universe.