Silly Dark matter question: why exactly is it necessary?

In summary, the conversation discusses the possibility of discrepancies in mass in our solar system being attributed to space junk and objects like cold neutron stars, but there is skepticism about this due to observational evidence for dark matter and predictions of BBN. The majority of dark matter is believed to be non-baryonic, and there are concerns about the presence of massive, dead stars in the halo. The post mentioned provides further information on observational evidence for dark matter and why it is believed to be in a non-atomic form.
  • #1
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Thinking about how little we know even about the amount of mass in our own solar system (Oort Cloud etc), could the discrepancies in mass simply be a ton of space junk (asteroids, planets etc) and even objects like cold neutron stars that are all very explainable objects in terms of their composition for the most part but are just really difficult to see?

Now don't get me wrong, I would find it highly improbable that the types of particles that we commonly interact with and form the spectrum of chemical elements that we are accustomed to is the only type of particular matter in the universe, or even the only type that can form larger "chemical systems". I am skeptical of our ability to account for all the mass of the common types of matter, how this non-luminous is distributed in the galaxy (are there some kind of galaxy formation dynamics that cause more or less of the non-luminous mass- relative to star material- to be distributed at the edge etc?)

I suppose I could keep going but I'm all questions and no answers. I'm just hoping someone can enlighten me to some of the stronger arguments for the existence of exotic dark matter vs. hard-to-see regular matter.
 
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  • #3
Another strong argument against the dark matter being made out of ordinary atoms is the predictions of BBN (Big Bang Nucleosynthesis). Basically, if the unseen matter were made out of baryons (i.e. ordinary atoms), then as the big bang cooled, nuclear fusion would have created a much higher number of heavy elements (He-4, D, He-3, Li) than we see. Since these nuclear fusion cross sections can be measured very accurately in the lab, we can predict very accurately how much of these elements there should be. If we assume the amounts of dark matter and ordinary matter given by the observations of the CMB, then the predictions of the abundances of the elements heavier than hydrogen agree very well with observations, which is a strong argument in favor of the whole model. Ned Wright has a really nice website on BBN here.
 
  • #4
The solar system is not a very good comparison since the mass of our solar system appears to be comprised almost entirely of ordinary, known objects like the sun and the planets and very little dark matter (I am uncertain if it is even detectable at all).

Most of the dark matter is in the halo of the galaxy and was discovered because of the relatively flat radial rotation curve of objects orbiting the center of the Milky Way.

Some dark matter is indeed ordinary baryonic matter like neutron stars and there are probably some astronomers who still believe in dark matter being comprised of MACHOS, but they are few and far between these days.

There are a whole lot of reasons for this skepticism. For instance, neutron stars are not truly "dark" and would be detectable due to gravitational lensing. Furthermore, since most of the dark matter is in the halo, we would need to explain why there would bee so many massive, dead stars in the halo and almost no low-mass, red dwarfs.
 
  • #5


I understand your curiosity and skepticism about dark matter. It is a complex and mysterious topic that has puzzled scientists for decades. However, there is ample evidence to support the existence of dark matter, and it is necessary to explain many observed phenomena in our universe.

First, let's clarify what dark matter is. It is a type of matter that does not interact with light or other forms of electromagnetic radiation, making it invisible to our telescopes. We know it exists because of its gravitational effects on visible matter. For example, galaxies rotate much faster than expected based on the visible matter they contain. This can only be explained by the presence of additional, unseen matter – dark matter.

Additionally, dark matter is necessary to explain the large-scale structure of the universe. Without its gravitational pull, galaxies and galaxy clusters would not have formed as we observe them today. In fact, simulations of the universe's evolution based on the amount of visible matter alone do not match the structure we see in the universe. Dark matter is needed to explain this discrepancy.

While it is true that there is still much we do not know about the distribution of mass in our own solar system and galaxy, this does not discount the need for dark matter. In fact, studies of the distribution of visible matter in our galaxy show that it cannot account for the observed rotation curves of stars. This suggests that there must be a significant amount of invisible matter present – dark matter.

Furthermore, the existence of exotic particles that do not interact with light or other forms of radiation is not a new concept in physics. Neutrinos, for example, are particles that have no electric charge and interact very weakly with other matter. They were once considered "exotic" but are now a well-established part of our understanding of the universe. Similarly, dark matter may also be composed of particles that we have not yet detected, but that does not discount its existence.

In summary, while there are still many unanswered questions about dark matter, its existence is supported by a wealth of evidence and is necessary to explain many observed phenomena in our universe. As scientists, we must continue to study and explore this mysterious substance to gain a better understanding of our universe and its components.
 

1. What is dark matter?

Dark matter is a hypothetical form of matter that is believed to make up about 85% of the total matter in the universe. It does not emit or reflect light, making it invisible and difficult to detect.

2. Why is dark matter necessary?

Dark matter is necessary to explain the observed gravitational effects on galaxies and galaxy clusters. Without it, the laws of gravity would not be able to account for the motions and structures we see in the universe.

3. How do we know dark matter exists?

Scientists have observed the effects of dark matter through its gravitational interactions with visible matter. This includes the rotation of galaxies, the bending of light around massive objects, and the distribution of matter in the universe.

4. What is the composition of dark matter?

The exact composition of dark matter is still unknown. It is believed to be made up of particles that do not interact with light, such as weakly interacting massive particles (WIMPs) or axions. However, these particles have not yet been detected.

5. How does dark matter relate to the Big Bang Theory?

Dark matter is thought to have played a crucial role in the formation of the universe after the Big Bang. It is believed that the distribution of dark matter helped to pull matter together, leading to the formation of galaxies and larger structures in the universe.

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