What is Dark Matter?

Most of the matter around us is invisible. Only its gravity can be felt. This mysterious substance known as Dark Matter is what astrophysicists and cosmologists have proposed to explain the strange movements of astronomical objects in our universe. What strange movements? What is this Dark Matter and where is it found? Are there other ways of explaining these oddities of the universe? In this video of Turtles All the Way Down we discuss these questions and more.
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***** CONTENTS **********
0:00 Introduction
0:47 Information from the Cosmos
1:45 Expected galactic rotations and movements
3:13 Mass of galaxies
4:34 Actual galactic rotations
5:15 Anomalies in movements
6:58 Categories of Dark Matter
7:55 MACHOs?
9:15 Neutrinos?
9:56 WIMPs?
11:02 Axions?
12:23 Modified Gravity?
14:00 Distribution of mass and energy in the universe
15:01 Conclusion

[@Nightscape_]

I'm going with glueballs being dark matter.

[@paxanimi3896]

What a clean explanation. Finally 👍

[@TurtlesWayDown]

Glad it helped!

[@michaelkahn8744]

I've been saying that "Dark Matter and Dark Energy don't exist" since 2022. We've spent almost a century to find Dark Matter and Dark Energy but still there's no sign of them. No matter what we try, a thing which doesn't exist can't be found. May be it is time to find an Alternative Way to Explain Dark Matter and Dark Energy. 4-D Hypershere model of Universe can easily explain Dark Matter, Dark Energy, Void and even the reason why the measurement values of Expansion Rate are around 70 km/sec-Mpc. Dark Matter, Dark Energy, Gravity, Void and Antigravity, ... all these are same phenomena. They just look different. I agree to the idea that the interaction between mass and space must be explained with quantum mechanics. But that doesn't mean gravity is the QM phenomena. That's because gravity is not a force. Dark Matter, Dark Energy, Gravity, Antigravity, Void... all these are just joint effects of the expansion of the Universe and the curvature of spacetime. Details are given below. Einstein’s theory of General Relativity states that spacetime is curved by the presence of mass. This curvature influences the motion other objects with mass and gives rise to gravitation. Thus, gravity is a result of geometric features in spacetime. However, we also observe gravitational effects - curvature of spacetime - in areas without any detectable mass. This has given rise to the concept of dark matter, which is matter that does not interact in any detectable way with normal matter, except through gravity. So, there is some large quantity of dark matter scattered throughout the universe, which curves spacetime and causes gravitational effects just like normal matter, but we cannot see or detect it with any known method. An alternative theory to the identity of dark matter is proposed - it is not matter at all, but rather an intrinsic curvature of spacetime. In other words, spacetime is not naturally flat. Even in the absence of matter, we observe some inherent curvature of spacetime. So, the question is now - why is spacetime naturally curved? Why is it not flat in the absence of mass? The universe is 4-dimensional, with 3 spatial dimensions and one dimension in time. Rather than consider time as a linear dimension, we can consider it as a radial one. Therefore, rather than describing the universe with a Cartesian coordinate system, we describe it with a 4-dimensional spherical coordinate system - 3 angular coordinates, φ1, φ2, φ3, and one radial coordinate in time, t. We live on the 3-dimensional surface of a 4-dimensional bubble which is expanding radially in time. Thus, the Big Bang represents t=0, the beginning of time. The crucial point is that the expansion of the universe is not homogeneous in all directions. The expansion rate at one point on the bubble’s surface may differ slightly from another point near it. The universe is only roughly spherical in 4 dimensions, the same way that the Earth is only roughly spherical in 3 dimensions. The same way we observe local mountains and valleys on the surface of Earth, we observe local “mountains” and “valleys” on the surface of the universe bubble. The inhomogeneity of the expansion of the universe has given rise to natural curvature of spacetime. This natural curvature causes the phenomenon of “dark matter”. “Valleys” in spacetime pull matter in, similarly to the warping of spacetime of massive objects. So “dark matter” is really “valleys” in spacetime that are expanding slower than the regions surrounding it. These valleys tend to pull matter in and create planets, stars, and galaxies - regions of space with higher-than-average densities of mass. Conversely, “mountains” in spacetime will repel matter away, an “anti-gravitational” effect, which gives rise to cosmic voids in space where we observe no matter. Each point on the surface of the universe bubble traces out a time arrow in 4-dimensional space, perpendicular to the surface. These time arrows are not parallel to each other since the universe is not flat. This causes points to have nonzero relative velocity away from each other. It is generally accepted that the universe is expanding faster than observable energy can explain, and this is expansion is believe to be still accelerating. The “missing” energy required to explain these observations has given rise to the theory of dark energy. The time dilation caused by non-parallel time arrows can be proposed as an explanation for dark energy. Alternatively, dark energy is real energy coming from potential energy gradients caused by non-parallel time arrows. As a sanity check, we can calculate the expansion rate of the universe based on the universe bubble model. Since the radius of the universe bubble is expanding at the speed of light in the time direction, it increases at 1 light second per second. Therefore, the “circumference” of the 3-dimensional surface increases by 2π light seconds per second, or about 1.88*10^6 km/s. This expansion is distributed equally across the 3-dimensional surface, so the actual observed expansion rate is proportional to the distance from the observer. At present, the age of the universe is estimated to be 13.8 billion years, so the radius of the universe bubble is 13.8 billion light years, or about 4233 megaparsecs (3.26 million light years to 1 Mpc). Thus, we can calculate the expansion rate of the universe, per megaparsec from the observer, as: Expansion rate = ((d(circumference))/dt)/radiusofuniverse=(1.88*〖10〗^6 km⁄s)/(2π*4233Mpc)=(1.88*〖10〗^6 km⁄s)/26598Mpc=70.82(km⁄s)/Mpc The popularly accepted empirical expansion rate is 73.5 +/- 2.5 km/s/Mpc, so our calculated value is close. There may be some additional source of expansion (or observed red shift) to make up for the discrepancy. For example, if two adjacent points have some gravitational gradient due to non-parallel time arrows, then light passing through these points will be red-shifted. - Cited from www.academia.edu/82481487/Title_Alternative_Explanation_of_Dark_Matter_and_Dark_Energy