“Topmost branches stretched in all directions, arched over the road, merging into a lush green canopy, dappled with yellow, saffron and vermillion. Bright-blue patches of sky peeked through the dome. A kingdom to itself, Danielle thought. Each tree was a fief, a world, or a galaxy, separated yet connected. The yellow and red leaves were stars, the green foliage encasing them was invisible dark matter.
“Could anyone guess the existence of green leaves, by looking at the bright leaves, stems and twigs?” she asked a nearby trunk.
It did not respond, but Danielle took the silence as “yes.”
“If stems and sticks connecting the leaves act like gravity, then counting their number in each tree will be like weighting a galaxy.”
From Initial Conditions
Danielle’s attempt to “visualize” dark matter is hardly scientific, but this is the problem with dark matter – it’s invisible. As the NASA site explains:
“We are much more certain what dark matter is not than we are what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the universe to make up the 27% required by the observations. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter. Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. High concentrations of matter bend light passing near them from objects further away, but we do not see enough lensing events to suggest that such objects make up the required 25% dark matter contribution.”
Another possibility cosmologists dabbled with (humorously summarized by XKCD):
So, in absence of direct evidence as to what dark matter is, physicists are free to go on to speculate what it might be. This video from BBC Future shows that even without it ever being observed, dark matter renders itself to “cinematic, mind-expanding video”.