As noted by others, what particular images look like will vary from monitor to monitor.
On my screen this photoset looks well shot, but a little dark. Checked it with Photoshop's histogram feature, which shows the range of pixel values in an image file, and its objectively too dark-- missing roughly the upper third of brightness values. In short its underexposed
So I lightened it Photoshop using the "curves" option, original on the left, lightened on the right
. . . the nice thing about Photoshop is that you can apply this kind of correction to an entire folder of images, which is what I'd do here, as all of them look similarly underexposed.