As an aside, you've put the cart before the horse here. When you have multiple competing theories, you don't study them separately. You design experiments which

As an aside, you've put the cart before the horse here. When you have multiple competing theories, you don't study them separately. You design experiments which will (hopefully) measure different results depending on which theory is true. This is exactly what is happening in cosmology. The experiments have ruled out MOND and their like. New results will rule out more dark matter candidates. Surprising results will lead to new theories.