When it comes to the cosmos, black holes are among the most intriguing and extreme entities. Formed by an overwhelming gravitational pull, they create regions in space where mass is so densely concentrated that even light cannot escape. This results in a zone of complete darkness where anything that enters is irretrievably drawn in, leaving us unable to investigate their internal composition. This leads to a pertinent question: could black holes be composed of dark matter? Radhana Domala poses this query on behalf of her child, who wonders:

“My son, who is in 3rd grade, is curious about dark matter and dark energy. He wants to know if black holes are made of dark matter, and I'm unsure how to explain it to him.”

You’ve come to the right place! Let's break this down in an accessible manner.

To begin, let’s clarify what it means for something to be “black” and if that equates to being “dark” as we typically use the term.

“Black” refers to a complete absence of color: any light that strikes a truly black object is entirely absorbed and not reflected. Such an object would simply retain the energy it absorbs, causing it to heat up.

However, even black objects radiate energy. For example, if you heat an iron pot, it will become hot and emit infrared radiation, eventually glowing red once it reaches about 525 °C (977 °F).

While black objects absorb all light and energy, dark objects absorb but do not emit detectable energy. True darkness means blocking all light without radiating any of its own.

Given these definitions, what exactly are black holes and dark matter? Are black holes truly black, and is dark matter genuinely dark?

Starting with black holes, they are areas of space where mass and energy are so densely packed that they create an event horizon— a boundary beyond which nothing can escape. Anything that crosses this threshold is pulled inexorably toward the central singularity, including light itself.

So, are black holes really black? Absolutely! They are among the blackest objects in existence. Though matter outside a black hole can emit light, the black hole itself does not. The only factor preventing them from being completely black is quantum physics, which still governs behavior even in empty space.

Near a black hole’s event horizon, space is severely curved, leading to the emission of a minuscule amount of radiation known as Hawking radiation, which is virtually undetectable. Therefore, while black holes are entirely black and dark, they do emit this tiny form of radiation.

Now, what about dark matter? How does it differ from “regular” matter?

When we refer to normal matter, we think of everything from planets to stars in our Solar System. If we only relied on visible light, most of the Universe would appear dark, with only a few sources of light visible, such as stars or light-emitting gas clouds.

True dark matter isn’t simply non-light-emitting normal matter. It’s a new kind of matter that doesn’t actually absorb light in the way we think. Instead of being “dark,” it should be termed “invisible matter” because light can pass through it unimpeded. This means that it doesn’t interact with light or normal matter in any discernible way.

The only reason we are aware of dark matter is its gravitational effects, which bend the space around it. As light from distant objects travels through this warped space, it appears distorted, revealing the presence of dark matter through gravitational lensing.

To understand how black holes form, we must gather sufficient mass in one area and allow gravity to do the rest. When enough mass is concentrated in one region, black hole formation becomes not just possible but inevitable.

This is commonly observed with normal matter, which can form black holes through several mechanisms:

• A massive star may undergo a core-collapse supernova, resulting in a black hole.
• Two neutron stars can collide, creating a black hole.
• A dense region of gas or stars can collapse directly into a black hole.

While theorized mechanisms exist for black holes to form from dark matter, they face significant challenges. Normal matter can collide and dissipate energy, enabling it to collapse. In contrast, dark matter does not interact in this way and cannot condense into high-density regions necessary for black hole formation.

Ultimately, while dark matter can contribute to a black hole’s mass once formed, it cannot initiate the process. Only normal matter, with its capacity for interaction and energy dissipation, can lead to the creation of a black hole.

Though it seems that dark matter cannot directly form black holes, some speculative theories suggest two potential scenarios:

1. The Universe may have originally contained regions rich in dark matter that collapsed into black holes, but these regions would need to be significantly denser than current observations suggest.
2. Dark matter could possess self-interactions that allow energy dissipation, although this theory would challenge current constraints.

In conclusion, while the answer leans towards “no,” the exploration of such questions can yield intriguing insights.

Feel free to send your "Ask Ethan" questions to startswithabang at gmail dot com!

Check out Starts With A Bang on Forbes, with articles republished on Medium after a week delay. Ethan has authored books like Beyond The Galaxy and Treknology: The Science of Star Trek from Tricorders to Warp Drive.