# Astronomy/Space queries

Question
Hey Courtney

No problem Dear. That was a difficult question on a field less known. However, I've another hypothetical question: do you feel that we know more than 10% of the span of the mighty universe? We have few details of the universe.

I'm not sure what you mean, so will give two possible answers.

(1) If you mean do we know what more than 10% of what is in the Universe is made of, the answer is probably no, since at least 90% of the mass is so-called dark matter, and we don't know what that is. I should point out that the term dark matter can be used in two ways. One refers to dark matter in galaxies and clusters of galaxies, all of which seem to have ten times more mass than can be attributed to visible stars (for more details, see "Dark Matter in Galaxies", at http://cseligman.com/text/galaxies/dark.htm ). That type of dark matter definitely exists, but we have no idea what it is, as the long discussion linked above explains. However, the term is also used to represent a presumably even greater amount of material of unknown amount and nature between galaxies and clusters, and that is what is usually under discussion when cosmologists talk about dark matter. Either way, dark matter makes up at least 90% and perhaps as much as 97% of the mass of the Universe, so as stated in the first sentence of this paragraph, we don't know about more than 10% (if that) of what is actually in the Universe.

(2) On the other hand, if you mean can we observe more than 10% of the actual extent of the Universe (that is, is the Observable Universe going out as far as the Cosmic Background more than 10% of the actual Universe), the answer is again almost certainly not. Estimates of how much larger than the Observable Universe the "Actual" Universe is range from a few tens to a few trillions of times larger diameter than what we can observe; but even the lower number would mean that what we can observe is only a tiny fraction of 1% of the actual Universe.

So (1) there is an immense amount of stuff all around us that we don't know how to observe, and though we know it is there we have no idea what it is. And although (2) the part of the Universe beyond what we can observe is probably not that much different from the part we can observe, there is an essentially infinite amount of it that lies beyond the furthest horizon we can ever observe.

Astronomy

Volunteer

#### Courtney Seligman

##### Expertise

I can answer almost any question about astronomy and related sciences, such as physics and geology. I will not answer questions about astrology and similar pseudo-scientific rubbish.

##### Experience

I have been a professor of astronomy for over 40 years, and am working on an online text/encyclopedia of astronomy, and an online catalog of NGC/IC objects.

Publications
Astronomical Journal, Publications of the Astronomical Society of the Pacific (too long ago to be really relevant, but you could search for Courtney Seligman on Google Scholar)

Education/Credentials
I received a BA in astronomy and physics and a MA in astronomy, both from UCLA. I was working on my doctoral dissertation when I started teaching, and discovered that I preferred teaching to research.

Awards and Honors
(too long ago to be relevant, but Phi Beta Kappa and Sigma Xi still keep trying to get me to become a paying member)