Which remarkable ancient civilizations are unknown to most people?

 The Mississippi Civilization.

This brilliant, highly developed—at least for its time and background—and urbanized civilization flourished between 800 and 1600 AD east of the Mississippi River in what are now the states of Missouri, Mississippi, Tennessee, and Illinois.
I would even go so far as to call them the "Mesopotamians of America."
Let me explain this in more detail.

1. Their way of life was almost exclusively urban.
Everything revolved around a life-giving river: the Mississippi.
They lived in and around huge urban centers, which were mainly oriented around ceremonial squares and temples.

Cahokia, the largest city discovered, was, for example, a huge center surrounded by small towns and villages - the suburbs.

The image below shows the city in question:
pyramid-shaped temples, the largest of which stood at the center of a central square, and small houses arranged around local squares.
Outside the city walls were other villages and towns, forming a suburb.
Cahokia was the largest city in North America around the year 1600 and was not surpassed until 180 years later by Philadelphia (with 40,000 inhabitants).

2. Their culture was conceived as a hierarchical system and oriented towards shared beliefs and values.
It appears that the Mississippians had a fixed clergy:
priests, executioners, and, of course, a large number of followers.

It is believed that the Mississippians (like their Nahua neighbors in Mexico) were involved in large-scale human sacrifices,
using beheading or strangulation.

From a sociological perspective, Mississippians constituted a civilization, as their society consisted of an institutionalized hierarchy and a complex chieftain system.
Furthermore, the dominant urban center (e.g., Cahokia) held a certain position of authority over its satellite settlements.
Finally, Mississippians were avid farmers whose lifestyle revolved around corn cultivation.

3. Finally, their civilization was prosperous enough to develop fine arts and pottery.
Mississippians were not only successful farmers, urban planners, and a highly organized society, but also artists.
Their artwork still reflects the uniqueness of their civilization today.

Finally, this wonderful civilization vanished for unknown reasons shortly before the Europeans arrived.
Frankly, that coincidence is pretty cool, to be honest.

To put it simply:
The Mississippians were a highly developed culture.
They were possibly the most advanced civilization in the pre-Columbian United States.
In any case, they were the most mysterious and least known civilization on the American continent.

The Milky Way is so vast, why haven't we discovered extraterrestrial civilizations yet?

 People really don’t understand how ridiculously big the universe actually is.

Look at this

That’s our solar system, right?

Well, no, not really. Our solar system actually looks like this.

Like if you went to Pluto and looked back towards the sun, that is pretty close to what you would see.

The nearest sun to us is Proxima Centauri.

Which is just around the corner at 4.2Light years. Why that’s practically in our back yard. Well, not really. That’s actually around 24,690,000,000,000 or 24.69 trillion miles.

Travelling at light speed would take you nearly 5 years to get there. But, since we can’t even come close to an appreciable fraction of light speed, we would need tens of thousands of years to get there.

The reality is that the miles (or Kilometers if you live in a non-stupid country) between solar systems are so insanely far that it is highly unlikely that any sort of biological life would have been able to get past them.

It is possible that we just don’t understand physics enough to make it happen. But it’s also possible that we just can’t go much faster than we can right now. And the reality of that is that we’re not getting off this rock. And neither is any Alien species getting off their rock.

How likely is it that there are alien civilizations in outer space? If they do exist, how did they originate?

 

We don’t know what the future holds for our species; if we or our descendant civilization exist for tens of thousands or millions of years, it’s possible that living on planets orbiting stars might, at some point, become obsolete. We might live some other way, perhaps just in space somehow. If we ever discover aliens, they might not need planets.

We know only one type of life on Earth, and we can only intuit that the first cells emerged similarly on other terrestrial worlds. Therefore, if aliens exist, they must have evolved similarly. Simple cells might have arisen in hydrothermal vents, and the first multicellular lifeforms might have existed in oceans. Aliens should also be terrestrial forms of fish-like ancestors that ventured on land 400 or so million years earlier.

Maybe multicellular, complex creatures must emerge on worlds orbiting stars similar in mass to the Sun. These systems have a limited lifespan before the star swells into a red giant that can engulf and destroy inner planets in its habitable zone. Aliens would know about it, and there might be no point in continuing to live on only their home planet, which is doomed to be destroyed, just as Earth will be engulfed by the hellish fires of our Sun in 4.5 billion years from now.

At a certain level of scientific sophistication, civilizations might establish space colonies in rotating rings that simulate the gravity of their home planet. Finding other worlds on which aliens would weigh similarly to where they came from might be difficult. This is the case in the Solar System, as the best planet we could ever colonize is Mars, but it might be too dangerous for us to live there for long, with just 38% strength of the Earth's gravity, and terraforming this red planet will never fix this issue beyond making the atmosphere more livable.

This is why terraforming worlds might be a waste of resources, and it’s just better to build habitats in space. At some point, aliens could be born in space for generations and forget their origin on a terrestrial planet. They can then more easily embark on traveling across the universe in their space homes, and this might also be our future. Who knows?