This is the "MACHO" (massive compact halo objects) theory, which has been tested and disproven. The missing mass can't be gas or dust clouds because that would

This is the "MACHO" (massive compact halo objects) theory, which has been tested and disproven. The missing mass can't be gas or dust clouds because that would have certainly observational traces which we have looked for and don't exist. It can't be planetoids because that would cause a frequency of gravitational micro-lensing of stars in other galaxies that we do not observe.