Because dark matter is the only theory that fits all of the observational evidence. There have been many theories put forward to explain the phenomenon which has been labeled "dark matter" and the only one that has withstood the test of time is "cold dark matter" in the form of "weakly interactive massive particles" (or WIMPs). Perhaps that theory is wrong, but so far it's held up remarkably well while other theories have crumbled.
Is it? Cosmologists constantly keep adding Dark Matter, Dark Energy and yet they can't explain things like the results of the experiment in this article. Could it be that the solution is much simpler? Could it be that we're simply misinterpreting or miscalculating some of the data?
Could it be that your understanding of the issues is simplistic, and scientists are actually asking all the relevant questions and dark matter is still the best answer they can come up with?
I'm going to second what gnaritas has said and suggest that perhaps you just have an insufficient understanding of the evidence and the current state of cosmological theory.
Also, despite the name "dark energy" has no relation to dark matter and the evidence for either does not overlap. Moreover, the inability of one theory to explain everything else in the universe does not invalidate that theory. The theory of biological evolution fails to explain the existence of neutron stars, yet it is still a valid theory.
Very unlikely. There have been a huge number of observations and a lot of eyes on the data. The evidence for something that behaves as if dark matter exists is overwhelming.
