What is this logical fallacy called?

On the surface this would appear to be appeal to authority . However, this usually requires the person to rely on a single expert, or a small group, since they're more easily swayed by personal opinion, and you can find a singular expert who agrees with you on a lot of things.

But, if you cite the fact that an overwhelming majority of experts on a topic believe something is true, that's actually a pretty good argument. Experts will almost always have access to more data, be better at interpreting it, and will have debated the merits of an idea before accepting it. It doesn't guarantee that the conclusion is true, but that the opinion is much more well-considered than a non-expert opinion ever could be.

Practically speaking, I think rejecting this argument will lead you to an incorrect conclusion more often than accepting it. You quite often see people who want to deny climate change, evolution, or the fact that the Earth is round, start off with this claim to get a listener to believe their conclusion is plausible. Then, they'll present distorted or cherry picked data that an expert would be able to pick apart, but a non-expert wouldn't necessarily have the knowledge to see through. Likewise, a non-expert who trusts expert consensus may not have a deep enough understanding to counter these arguments.

Whether or not the person making the claim supplies evidence or not, if it's true that thousands of scientists have been saying X for however many years, there will be a case put forward by the scientists for that claim.  However, if there is a minority of scientists and other independent researchers who refute the claim, their refutation needs to be addressed.

This is where the term "conspiracy theorist" is exposed as so terribly wrong. Scientists who challenge what they believe to be scientific dogma are not "conspiracy theorists", they are scientists challenging what is put forward as science and they may not believe any conspiracy has actually occurred in what they believe is false having come to be accepted, they may think it's just somehow become accepted because a certain person or group of people's thinking on a subject has dominated and taken hold. And even if they do think the dogma is the result of a conspiracy that is not their main concern, their concern is only to correct what they believe is wrong.

Thank you, Mchasewalker!

You bet, blood!

There was a time, for example, when “all” the scientists believed the world was flat and the sun revolved around the earth. You would be imprisoned or killed to suggest otherwise. 

I think the notion that "all" scientists ever believed anything isn't perhaps quite true, eg, Philolaus in the 4th century BC apparently worked out heliocentrism and I remember casually picking up a book on scurvy and being struck by the author's claim that the cure for scurvy was long-known before it became recognised generally. It's quite possible, I'd imagine, that heliocentrism preceded geocentrism which, if so, is rather remarkable: the truth is determined, superseded by a lie, and then re-determined. 

Since the advent of covid I've been astonished by the wealth of refutational material I've discovered on virology and vaccinology and that criticism of both Pasteur and Jenner started from the get go. However, this criticism is simply not well-publicised.

Lies, damn lies, and scientific research. by Lancet editor, Michael McCarthy
https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(04)17334-0/fulltext

Louis Pasteur's notebooks, long kept secret, reveal that he misled the world and his fellow scientists about the research behind two of his most famous experiments: the vaccination of sheep against anthrax, and that of a boy against rabies.

You really do have to wonder about a field of science whose "father" is a recognised fraud.

In the 25 works shown here in Amazon almost half were published in the 1800s.
https://www.amazon.com/History-of-Vaccination-8-book-series/dp/B085CBC263

Analyst, Mike Stone, provides another explanation for the "eradication" of smallpox that I find compelling, namely that smallpox didn't have a distinctive symptomology and where the symptoms once ascribed to smallpox have manifested since its alleged eradication, other conditions are ascribed such as monkeypox.
https://viroliegy.com/2022/01/05/was-smallpox-really-eradicated/

Technically, of course, you are correct. I was trying to illustrate a point. In the future, in making such a point, I'll re-phrase it to something like the "leading scientists," the "popular scientists," or the "accepted scientists." Maybe you have a better way to put it. I so, I'd love it. 

Perhaps "scientific dogma"?

I looked up a bit about heliocentrism and I found this thread on Quora quite fascinating:
https://www.quora.com/Was-Galileo-unable-to-actually-prove-his-heliocentric-theory/answer/Jason-St-Pierre-3

Another interesting thread on its history:
https://www.quora.com/What-was-the-heliocentric-theory-and-who-developed-it/answer/Gavin-Kanowitz

I found another thread but can't re-find it where the writer said that the reason Copernicus is accepted more as the discoverer of heliocentrism than the much earlier scientists is that he had mathematics to explain it better - but obviously he didn't have a full explanation either. I'd imagine that dogma will tend to survive more easily where its refuters don't have a robust theory to replace it. While I can certainly see the holes in the science of germ theory, for example, there is no clear theory to explain conditions currently thought to be caused by viruses.

I fear I do not understand the relevancy to my point. Sorry.

[To Dr. Richard]

Sorry, Dr Richard, just the first sentence is my response (Perhaps "scientific dogma"?) - a suggestion instead of "leading scientists", etc.

The rest is just an FYI - links to what I find interesting on the subject of heliocentrism.