Nebulae are an argument in favor. They are observable b/c they're not dense. They have a very high surface area to mass ratio. Light passes through them and th

Nebulae are an argument in favor. They are observable b/c they're not dense. They have a very high surface area to mass ratio. Light passes through them and they have absorption spectrum. They also emit a lot of black body radiation. By contrast a moon size object has a very low surface area to mass ration. It has no absorption spectrum b/c light can't pass through - and any black body radiation would be negligible b/c of relatively small surface area