Default alignment for new blocks

I doubt that there is one alignment for all the blocks, to use as default. And it wouldn’t make sense, really, because the most used block is paragraph, which shouldn’t be aligned at all. As a matter of fact, I think you are seeing the default “no alignment” as something it is not. In HTML, block elements have a default of 100% width, and if you center them, you can’t tell because there is no extra space to put on either side.
Of course, I could be mistaken, and your theme or a plugin might be manipulating the default value.

My opinion: centered images can look nice, but if it’s text, it should be a user action to center it, because centered text is harder to read.

Thanks for the reply @joyously .

I was wrong in stating that it applies to all sorts of blocks.
I was referring to the action of adding a new block in the editor. By default image blocks take the full width of the editor, paragraph blocks however don’t. They’re centered by default. The text is not aligned in the center but the block only takes up the central space of the editor leaving a white column on the left and the right.

I would like to know if there’s a way to have the image block act in a similar way as the paragraph block, in other words have them appear centrally by default.

It would also help for all the content we still have to migrate from the classic editor to the block editor. All pictures are suddenly misaligned now.

Ah, this sounds like your theme is styling things differently for blocks than for legacy HTML. The wide and full alignment options are not part of the editor (the interface is there only if the theme opts-in), so perhaps the default is handled at that time.
You can ask at your theme’s support forum.
You can try going into Customizer and previewing a different theme, to see if it handles legacy and block content the same way.