Why does the dark matter ratio differ between the universe and the Milky Way?

[RyanH42]

We know that Ω_DM=0.26
and Ω_BM=0.46 which this leads us this result Ω_DM=5Ω_BM
So In universe If there's m baryonic matter there will be 5 times more dark matter (5m).This is true in larger universe distances

Milky way galaxy mass is 9x10¹⁰ solar masses but dark matter value is nearly 20 times bigger then that (average of 6x10¹¹ and 3x10¹²) so here Ω_DM≅20Ω_BM

So universe contains 5Ω_BM
and our galaxy contains ≅20Ω_BM.

Is that mean some places there's no DM nearby the galaxy or not nearby galaxy places ?

 

[michael879]

Our galaxy is actually a fairly small scale when you're talking about total densities of the observable universe. The matter distribution of the universe doesn't become homogenous until MUCH larger scales. Galaxy clusters are more representative structures for the rest of the universe, as over 90% of the baryonic matter is gas within the IGM. At the galactic scale most of the baryonic matter is from stars

I believe the answer you're looking for is that yes, outside of galaxies the ratio of DM/BM is slightly lower than average. It makes sense if you consider how lambda CDM predicts that DM is key to the formation of galaxies

 

[ohwilleke]

We know that Ω_DM=0.26
and Ω_BM=0.46 which this leads us this result Ω_DM=5Ω_BM
So In universe If there's m baryonic matter there will be 5 times more dark matter (5m).This is true in larger universe distances

Milky way galaxy mass is 9x10¹⁰ solar masses but dark matter value is nearly 20 times bigger then that (average of 6x10¹¹ and 3x10¹²) so here Ω_DM≅20Ω_BM

So universe contains 5Ω_BM
and our galaxy contains ≅20Ω_BM.

Is that mean some places there's no DM nearby the galaxy or not nearby galaxy places ?

To be nit picky, only because the number is so critical to your analysis, you mean to say that Ω_BM=0.046.

More generally, there are well established phenomenological relationships between the BM in a galaxy or galactic cluster defining total mass as a function of a single BM variable (with a kinked curve that breaks with different galaxy types) in any given system for the entire universe, subject to only modest random scatter. Matter, BM and DM alike, is overwhelmingly clumped in galaxies and galactic clusters which are strongly clumped in huge filaments of matter between large voids which very low densities of matter by comparison.

Thus, the BM/DM ratio is largely a function of the distribution of galaxies and galactic clusters by size in the universe. As very crude a rule of thumb, smaller galaxies tend to have higher DM/BM ratios than larger galaxies (at least within a given type of galaxy and comparing the mean for different types of galaxies), but clusters have quite high DM/BM ratios. The universe's DM/BM ratio of 5 corresponds to a weighted mean galaxy size in the universe corresponding to the mass that has a DM/BM ratio that is typically about 5. A typical elliptical has about a 2-1 ratio of DM-BM, while a typical spiral galaxy like the Milky Way is closer to 20-1 or 12-1. A 5-1 ratio seen in the universe as a whole is at the low end for spiral galaxies (and thus associated with larger spirals with bulges) and at the high end for elliptical galaxies (and associated with smaller elliptical galaxies that aren't perfectly spherical).

 

[Chalnoth]

So universe contains 5ΩBM
and our galaxy contains ≅20ΩBM.

As I understand it, it's probably mostly a feature of galaxy formation. When the stars first turn on in smaller galaxies, a lot of the normal matter gets expelled from the galaxy. Larger galaxies have stronger gravity, which prevents most of the normal matter from escaping, but some still does.