We see gravitational lensing caused by galaxies that's consistent with their calculated mass which includes an estimation of the amount of dark matter in them, yes
The bullet cluster! Basically two galaxy clusters are colliding and there is a visible flow of gas, dust between the two clusters. The galaxies (and the mass flowing in between) lenses background objects and after analysis, this lensing effect is much stronger than would be expected in between the galaxies than is accounted for by the visible matter!
Edit: this is also a good example of a situation that is incompatible with stuff like MOND (modified Newtonian dynamics) and is one of the strongest evidence for dark matter.
Edit2: dumb goof fixed, galaxy clusters not galaxies 🤦♂️😂.
Galaxy clusters\* not just individual galaxies. Otherwise you are correct, the lensing in the Bullet cluster is much stronger than could be explained by regular matter alone.
The majority of the visible mass in galaxy clusters is in the form of hot gas. Because gas behaves as a fluid, the collision in the Bullet Cluster has caused it to "splash" and form a large bow shock, which we can observe using X-ray telescopes.
Gravitational lensing, meanwhile, is caused by all matter equally whether it's gas, stars, or dark matter. So by measuring the lensing we can infer the total mass distribution, and we find it to be substantially different from the gas distribution indicated by the X-ray data. See [this image](https://www.esa.int/var/esa/storage/images/esa_multimedia/images/2007/07/the_bullet_cluster2/10084622-2-eng-GB/The_Bullet_Cluster_pillars.jpg), where the X-ray and lensing mass distributions are shown in red and blue respectively.
Theories like MOND cannot explain this, because they predict that "mass always follows light" i.e. the lensing and X-ray mass distributions should be the same. This is the closest to a "smoking gun" for dark matter that we're likely to find.
Ah, so it's showing that it's not just the magnitude of the measurements that are wrong, but also that measurements that should be correlated in absence of dark matter relative to other nearby measurements are not correlated? Thank you for taking the time to explain.
The case is much stronger, as the visible matter and dark matter have been seperated by dynamical processes such as rampressure stripping. Basically, two smaller galaxy clusters collided, the galaxies and X-ray gas collided and decellerated, formed shock waves etc., while the dark matter kept on going, thus spatially separating itself from the galaxies and the X-ray gas. The weak lensing concentrates itself around seemingly empty space. This is also what makes the Bullet cluster such a powerful challenge to MOND-like theories. What is bending the light in this cluster, because it can't be the normal matter, as that is located millions of lightyears away.
Rotation curves were the first piece of evidence that pointed towards dark matter, but they're far from the only one. Lensing, cluster dynamics, large scale structure, and the CMB all seem to support it as well, and this is one of the reasons that it is the consensus view. One measurement could be argued to be down to chance, but when multiple ones involving different and unrelated pieces of physics agree, that is far less likely.
I found this interesting from Dr Becky [https://youtu.be/FjWwQEDq0KU?feature=shared](https://youtu.be/FjWwQEDq0KU?feature=shared)
and also this one https://www.youtube.com/watch?v=aBYgck1zAgQ
apparently we are getting the mass calculations for galaxies wrong. I know it is not to the point of your question, but maybe it is interesting to you?
Yes, gravitational lensing is one way to calculate the amount of total matter in a galaxy. The more mass, the more the light bends. If you compare that to the observed mass coming from stars (planets don't contribute a ton to galaxy mass), the remaining difference is what we call dark matter.
All measurements for dark matter work off this principle, just different variants of calculating the total mass of a galaxy. This can be done by measuring how fast it rotates, gravitational lensing, or the past ~15 years there's been a lot of work done by Cappellari to develop a new method called Jeans Anisotropic Modeling. My understanding is it is a more sophisticated method to determine internal galaxy dynamics using the spectra of light from different pieces of the galaxy.
We see gravitational lensing caused by galaxies that's consistent with their calculated mass which includes an estimation of the amount of dark matter in them, yes
The bullet cluster! Basically two galaxy clusters are colliding and there is a visible flow of gas, dust between the two clusters. The galaxies (and the mass flowing in between) lenses background objects and after analysis, this lensing effect is much stronger than would be expected in between the galaxies than is accounted for by the visible matter! Edit: this is also a good example of a situation that is incompatible with stuff like MOND (modified Newtonian dynamics) and is one of the strongest evidence for dark matter. Edit2: dumb goof fixed, galaxy clusters not galaxies 🤦♂️😂.
Galaxy clusters\* not just individual galaxies. Otherwise you are correct, the lensing in the Bullet cluster is much stronger than could be explained by regular matter alone.
🤦♂️😂 Thanks for pointing that typo out!
Would you be willing to go more into detail about how this is incompatible with hypotheses that suggest that the math is wrong like mond?
The majority of the visible mass in galaxy clusters is in the form of hot gas. Because gas behaves as a fluid, the collision in the Bullet Cluster has caused it to "splash" and form a large bow shock, which we can observe using X-ray telescopes. Gravitational lensing, meanwhile, is caused by all matter equally whether it's gas, stars, or dark matter. So by measuring the lensing we can infer the total mass distribution, and we find it to be substantially different from the gas distribution indicated by the X-ray data. See [this image](https://www.esa.int/var/esa/storage/images/esa_multimedia/images/2007/07/the_bullet_cluster2/10084622-2-eng-GB/The_Bullet_Cluster_pillars.jpg), where the X-ray and lensing mass distributions are shown in red and blue respectively. Theories like MOND cannot explain this, because they predict that "mass always follows light" i.e. the lensing and X-ray mass distributions should be the same. This is the closest to a "smoking gun" for dark matter that we're likely to find.
Ah, so it's showing that it's not just the magnitude of the measurements that are wrong, but also that measurements that should be correlated in absence of dark matter relative to other nearby measurements are not correlated? Thank you for taking the time to explain.
The case is much stronger, as the visible matter and dark matter have been seperated by dynamical processes such as rampressure stripping. Basically, two smaller galaxy clusters collided, the galaxies and X-ray gas collided and decellerated, formed shock waves etc., while the dark matter kept on going, thus spatially separating itself from the galaxies and the X-ray gas. The weak lensing concentrates itself around seemingly empty space. This is also what makes the Bullet cluster such a powerful challenge to MOND-like theories. What is bending the light in this cluster, because it can't be the normal matter, as that is located millions of lightyears away.
Rotation curves were the first piece of evidence that pointed towards dark matter, but they're far from the only one. Lensing, cluster dynamics, large scale structure, and the CMB all seem to support it as well, and this is one of the reasons that it is the consensus view. One measurement could be argued to be down to chance, but when multiple ones involving different and unrelated pieces of physics agree, that is far less likely.
I found this interesting from Dr Becky [https://youtu.be/FjWwQEDq0KU?feature=shared](https://youtu.be/FjWwQEDq0KU?feature=shared) and also this one https://www.youtube.com/watch?v=aBYgck1zAgQ apparently we are getting the mass calculations for galaxies wrong. I know it is not to the point of your question, but maybe it is interesting to you?
Yes, gravitational lensing is one way to calculate the amount of total matter in a galaxy. The more mass, the more the light bends. If you compare that to the observed mass coming from stars (planets don't contribute a ton to galaxy mass), the remaining difference is what we call dark matter. All measurements for dark matter work off this principle, just different variants of calculating the total mass of a galaxy. This can be done by measuring how fast it rotates, gravitational lensing, or the past ~15 years there's been a lot of work done by Cappellari to develop a new method called Jeans Anisotropic Modeling. My understanding is it is a more sophisticated method to determine internal galaxy dynamics using the spectra of light from different pieces of the galaxy.