[00:00] Physicists believe that 80% of the matter in  the universe is ‘dark matter’ that we can’t   see. But despite decades of trying to measure  even a single one of the dark matter particles,  
[00:12] they haven’t been able to confirm that this  matter really exists. In the past years,   physicists have been rethinking the dark matter  idea… but not in the way you might expect.
[00:25] Dark matter has made physics a laughingstock  because it doesn’t seem like a scientific idea.   Hey, we have these observations that we don’t  understand, let’s just make up some invisible   stuff so that the maths works out! In every-day  circumstances making up invisible stuff to  
[00:41] explain observations you don’t understand  is a fast track to psychiatry. However,   in science it can be justified provided that  the explanation is simple. Because if you can   calculate a lot of observable consequences  from a few assumptions that is good science.
[00:58] And dark matter used to be such a simple,  good scientific explanation. That’s because   for astrophysicists “dark matter” doesn’t just  mean invisible stuff, it means very specifically  
[01:10] something that doesn’t interact with light and  that is fully described by its energy density,   without pressure. Dark matter in its original form  really only had this one parameter, the initial  
[01:22] energy density, and then all the rest should  have followed from that by the laws of gravity. Problem is that this didn’t work.  The observations just didn’t comply,   and the more observations astrophysicists  gathered the less it was working. You see,  
[01:36] if you stick with the simple dark matter model,  that tells you what galaxies should look like   and how they form as matter in the universe  clumps. This is because there is so much more   dark matter than normal matter, that the dark  matter totally dominates what is happening.
[01:52] But the predictions from the simple dark matter  just don’t fit to observations. Big galaxies seem   to have fewer small galaxies around them than  predicted, the density in galactic cores isn’t  
[02:05] as high as predicted, galaxies form faster than  expected, and their size is correlated with the   brightness which really shouldn’t be the case.  There are new problems popping up each month,   like this case that just doesn’t want  to fit into the simple dark matter mode.
[02:21] So what have astrophysicists done in response to  these observations? They have made the models more   complicated, and they have made many, many  of these models. This is why there are now  
[02:33] several headlines each week about some new dark  matter ideas. “A New Theory of Dark Matter Could   Solve Three Cosmic Mysteries,” or “Could dark  matter be made of black holes from a different   universe?” or “Scientists Are Pretty Sure They  Found a Portal to the Fifth Dimension” , that  
[02:49] portal being dark matter. Or “We still can't  see dark matter. But what if we can hear it?”   These are all headlines about increasingly  obscure and complicated dark matter models. The one big trend has been to go from  one dark matter particle to an entire  
[03:05] “dark sector” of multiple particles, sometimes  these are literally hundreds. The other trend   has been to give the dark matter interactions,  this is known as self-interacting dark matter.
[03:17] Just have a look at these recent arxiv  posts with the keyword “dark sector”.   Primordial black holes coupled to a dark sector,  a massive vector sector, dark sector antibaryons,  
[03:29] a massive dark photon and dark matter particles of  spin up to 1, Higgs portal, heavy neutral leptons,   and heavy QCD axion models, Decaying spin-3/2  dark matter, two scalar fields, an anisotropic  
[03:43] spinor field, a sequestered dark sector. And  those are just the listings FROM THE PAST WEEK! These papers are all a 10 out of 10 on  the bullshit meter. Oh, you might say,  
[03:55] that’s unkind. Maybe it is. But this is not proper  science. These are people producing papers with   bullshit theories that they fumbled together so  that they fit observations and can’t be ruled out.
[04:09] It’s no coincidence that this is the same nonsense  theory-production which happened two decades ago   in high energy particle physics, because people  left particle physics and now do astrophysics,  
[04:23] but it’s the same damn thing. They  have not learned anything. Instead,   they complain about me when I point out  they get paid for producing bullshit papers.
[04:35] Why am I telling you about this? It’s so that  you know why I don’t talk about the papers that   these headlines are about. They have nothing to  do with reality. There’s nothing interesting to   find there. What astrophysicists should do is  to collect all data and look for the SIMPLEST  
[04:51] explanation that explains it, systematically. We  already know that this will be a combination of   cold dark matter and modified gravity. But  that’s the path no one wants to go down.
[05:03] So, yes, astrophysicists  are rethinking dark matter,   but the result is that now  it makes even less sense. Science needs problem solvers, people like  you. If you want to turn your talent into  
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[06:09] annual premium subscription with unlimited  access. Thanks for watching. See you tomorrow.