I'm not the one saying this, it's NASA - and with it's publicly available research findings - plus the scientific community all over the world. Let's try and understand this. What I'm talking about is on this link
https://map.gsfc.nasa.gov/universe/uni_shape.html
What they mean is, except for a 0.4% marginal error, the universe is FLAT!!
Flat as in a sheet of paper? That's what they are saying. Flat as in, it is finite, or measurable, or has some sort of boundaries, along the z - axis, and it is infinite along the x and y axes. Though the dimensions of this z - axis, the thickness of the sheet of paper, isn't calculated yet.
How did they calculate this?
The answer is WMAP or Wilkinson Microwave Anisotropy Probe. Let's see what the WMAP means, word by word. More info on WMAP from NASA is at this link.
https://map.gsfc.nasa.gov/news/facts.html
I sure don't need to explain a probe, it's plain english. And I'm pretty sure you don't need an explanation on Wilkinson either, but just in case. He was a pioneer cosmologist, you can check his wikipedia page if you wish.
https://en.wikipedia.org/wiki/David_Todd_Wilkinson
The two terms we need to understand in WMAP are Microwave and Anisotropy.
Anisotropy: we can already see, it means something opposite to what isotropy means. So if we get the meaning of isotropy, we also get anisotropy. Now isotropy has many specific definitions in many fields including maths, physics, biology, and geography. But in all these fields it only has something to do with being uniform. As in having a uniform geometry in maths, or uniform distribution in physics, or cells showing more or less uniform characteristics every where in biology, etc.
Isotropy is about uniformity of a property in every dimension (or simply direction, or place it exists in). This makes our property uniform everywhere, and hence, independent of dimensions.
So, Anisotropy, obviously means something that is not uniform. Or by definition here, it is the changing (dependence) of some property with dimensions (or directions or places).
So what is changing here? The microwave radiation. Let me tell you a story about this.
In 1937, an engineer in bell laboratories, was studying the radio waves. While studying these, he found radio waves that were coming from space. And then people started building radio telescopes to see where they came from.
Optical telescopes were already being used, and many galaxies around us were discovered. And so was the huge gap between the galaxies, and also even larger gaps between galaxy clusters.
Using one of these telescopes - must have been a quite sensitive one in those days - two radio astronomers discovered a faint noise signal, which also showed as a faint glow, and which wasn't associated with any star or galaxy or any other object. And hence, the Cosmic Microwave Background (CMB) radiation was discovered. These two dudes got Nobel prize for this discovery.
https://en.wikipedia.org/wiki/Cosmic_microwave_background
This radiation, seemed "almost" isotropic (uniform). And it is the change in the uniformness (anisotropy) of CMB radiation, that WMAP has studied.
Don't worry about the microwave, it's just a name given to a range of frequncies in cosmic radiation.
The data that was collected from WMAP, was astonishing, and perhaps the most important data that was ever collected on the origin of the universe, And I'll post a link about it below, and hence conclude our post. More details coming soon.