For example, he rejects the idea that gravity bents light and that black holes exist. He wrote a book predicting the LHC would fail to find the Higgs boson, and doesn't accept that what they found is indeed the Higgs. He hates the idea of a graviton enough to call for defunding of LIGO! He doesn't believe that Hubble's law is caused by the expansion of space. He also thinks dark matter is just some mistake.
In episode 446 Justin Jackson says,
I wouldn't be surprised at all if 5, 10, 20 years from now, we discovered the universe isn't expanding, and that it's not 13 billion years old, but infinitely old, that most of the dark matter is not actually matter... the point is that what we do know is based on a smaller observation than we're gonna have in the future...This finger-crossing shows how his denial comes from misunderstanding the scientific process.
Let's start with his claim that future observations could do away with a finite, expanding universe.
Just as natural selection is the only theory that can explain the origin of species, the Big Bang theory leaves no room for other explanations for the observed expansion of the universe. However, there are several kinds of theories that attempt to do away with or get around the Big Bang singularity. Notice that these are extensions of the Big Bang theory, all of which contain the expansion of spacetime from a very dense beginning a finite time in the past.
First, you can play with coordinate systems to make the singularity go away, but you still have a finite time to the edge of your coordinate system. These get a lot of press because a universe without a big bang singularity sounds impressive. Never mind that it's just a mathematical trick. Anyway, this certainly isn't what Justin is talking about, because its a matter of theoretical interpretation, not future observation.
Second, there are cyclical cosmologies which have our big bang not as an origin of time, but as a boundary condition to causality in one iteration of an infinite series of universes. True, this sort of theory could be favored by future observations, but Justin wouldn't be satisfied by this, either.
Justin argues for a steady state universe where space isn't expanding from a bounce or bang. He dismisses the cosmological redshift as some trick of optics. He can't accept that it's a consequence of the Doppler effect due to the apparent velocity of objects in an expanding spacetime.
No new evidence can ever be discovered that is consistent with this steady state fantasy. Future measurements of stars aren't going to suddenly show a reverse in Doppler shift, so that when added to previous observations, the shift averages out to zero. That may have been conceivable to Milton Humason when he first sat down to start comparing the distances of the galaxies that he had measured with their radial velocities. However, all hope for the steady state theory were dashed the moment he analyzed a statistically significant number of galaxies. Then came Penzias and Wilson...
Justin also rejects the fact of the acceleration of the expansion of the universe. There are competing theories to explain this fact, like the Cosmological Constant, which may or may not be correct. But just as before, there has been no hope that future observations might contradict the truth of this fact since Saul Perlmutter, Brian Schmidt and Adam Riess received their Nobel prize. No future observations of stars can ever reverse the statistical trend showing an increased in the rate of expansion of the universe. That kind of thing doesn't happen in science because it doesn't happen in nature.
I don't know what he means by dark matter not actually being matter. There's missing gravitational mass/energy needed to explain galaxy rotation, velocities of galaxy clusters, gravitational lensing observations, and anisotropies in the CMBR. All of these lines of evidence point to the the same amount of missing mass/energy, acting like a cold gas. It might be made of WIMPs, or it might be something else. Either way, if cold gas carrying mass/energy counts as matter, then it's matter.
Justin isn't really concerned with the definition of matter. He just doesn't believe that the evidence for dark matter requires new physics. He thinks it's all a big mistake, or something.
From personal dislike for these theories, he rejects mountains of clear and unambiguous evidence.
That must take an astronomical level of denial.