Dark matter is not an assumption. Dark matter is not a hypothesis. Dark matter is not a theory.
Dark matter is a series of observations of the universe. Galaxies spin are observed to spin differently than our models and estimates of their mass say they should. Velocities of galaxies in galaxy clusters are much faster than the sum total of the gravitational effects of the cluster can account for. The bullet cluster lenses gravity in a distribution that is not in accordance with the matter that we see. The CMB (cosmic microwave background) is lumpy, too lumpy for our models. This is dark matter; dark matter is a series of observations where the stuff we see does not line up with what our models predict. Dark matter is not an assumption. Dark matter is opening our eyes and looking at the sky.
Now, we can have different theories of dark matter. Hypotheses or theories that attempt to explain the observations. Currently the leading theory is WIMPS, but MACHO and MOND were in the running for a while.
By way of analogy, we have known that light was a thing for thousands of years. Light is not an assumption. Light is not a hypothesis. Light is not a theory. Light is the observation that we are able to see. Light is the observation that we see better when the Sun is up than when a full Moon is up, and sometimes barely at all if there's a new moon or if we're in a cave. Light is the observation that the Sun is brighter than the Moon. Light is the observation that we can make a fire, perhaps a campfire, or a candle, or a torch, that can enable us to see in the dark. Light is the observation that if we put the fire out, we can't see anymore. There were several theories that tried to explain what light is; the Greek theory about our eyes sending out feelers, or waves in the luminiferous aether, or a stream of billiard ball-like particles, or waves in the electromagnetic field. We can have a meaningful discussion about which of these theories is the best one, but we can't have a meaningful discussion about whether the phenomena known as "light" is an assumption: We can see. Therefore light, whatever it happens to be, does exist.
Dark matter is no different.