Denmark has one culture. America has multiple cultures, each one indigenous to its region. And new cultures keep coming, and it takes time for them to earn a place in the indigenous culture of that region. But that’s possible only with limited, vetted immigrants, over an extended period of time. At first there’s friction, discrimination, disdain, and slowly, maybe after a generation or two, the newcomers find their place. A pell-mell invasion doesn’t work that way. It’s a hostile takeover, and an intentional erasure of the indigenous cultures of all regions affected.
The US does have one big culture as well, most Americans do not see it because they are in it. Once you live outside the US for some years American Culture slaps you hard in the face when you come back. You see it. And, despite what some people cry about, it actually is a pretty nice culture.
Inside the US are a bunch of mini cultures, more then in Denmark because we are bigger but the biggest divide is still, "Country vs City". My area which is semi rural has more in common with all other rural areas in the US then it has with any city areas in the US. However we still have more culture in common then we do with Canadians.
Most people think of national culture as food, dress or language. It is not. It is attitude and the actions that come from those attitudes. In the US when I see someone pulled over and there is not a tow truck it it the culture to stop and see if you can help. In Mexico you do not do this, it is not the culture. In the US you see people out in public and they are not dressed for high tea you politely leave them to it. In certain cultures the response would be to call your buddies in for a gang bang.
Currently we (by that I mean the west in general) have been importing people who do not share our culture. So this is not going to end well for anyone.