Dark Matter
I see no mystery over dark matter.
Scientists find that the Mass of the Universe is greater than what has been observed. They call that unobservable mass - dark matter.
Although dark matter has not been directly observed, its existence and properties are inferred from its gravitational effects such as the motions of visible matter, gravitational lensing, its influence on the universe's large-scale structure, on galaxies, and its effects on the cosmic microwave background.
~ Wiki
I read an article about a new galaxy Hubble located in a gravitational lensing arc.
http://www.sciencedaily.com/releases/2017/07/170706155944.htm
Consider dark matter for a moment...
It is the catch-all phrase that suggests that there is more matter in the Universe than we can detect. well, duh.
Our eyes are detectors. We see the moon. There are times when we don't see the moon because it is in Earth's shadow or obscured by clouds. We know it is there because we see the affect it has on tides and stuff. In this sense, the moon is dark matter some of the time.
Now, think about all the planets, asteroids and gas clouds in the Universe. We detect planets around stars by measuring the effect it has on the stars luminosity. We measure the dip in brightness to determine its presence. We measure the gravitation wobble of the star's placement in the sky.
If an moon is behind a planet and our view never changes angle we will never detect that moon. If an asteroid is heading for Earth and it is being followed by more asteroids directly behind it we will never detect they other asteroids unless we look at it from a different angle.
Gravitational lensing allows us to see around a star or galaxy because light is bent by matter. It is only bent by a small degree considering the size of space. What about all the matter behind that star that did not get bent to our detectors? Only some of the light gets bent. Only a small fraction of all the light gets bent and hits our detectors. We are likely missing a view on complete galaxies.
We also see galaxies because they are positioned in a way that allows them to register. Hold a needle edge on and look at it thru binoculars from 1,000 yards and see how much you can see. If you can see anything you might see a point. The needle could be a mile long but all you see is the point. Until, You change your observation angle.
Consider that stars are seen by the light and energy they release that hits our detectors. We see other celestial objects from the reflected light, as an empty spot against a background or by inferring gravitational influence.
Space is big, really big. There is a lot of stuff out there that we can't detect. Dark matter is a term used to describe that inability to detect. It is a name used as a placeholder not an exotic form of matter.
The standard model of cosmology indicates that the total mass–energy of the universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. Thus, dark matter constitutes 84.5% of total mass, while dark energy plus dark matter constitute 95.1% of total mass–energy content.
~ wiki again
All that tells me is that we can only detect about 5% of the Universe. Imagine if you had to live with only 5% of your senses.
Science know a lot about the Universe but if we are only detecting 5% of the whole, we don't know Jack.