Is space allowed between #! (shebang) and /bin/bash?

Yes, this is allowed.

The Wikipedia article about the shebang includes a 1980 email from Dennis Ritchie, when he was introducing kernel support for the shebang (as part of a wider package called interpreter directives) into Version 8 Unix (emphasis mine):

The system has been changed so that if a file being executed begins with the magic characters #!, the rest of the line is understood to be the name of an interpreter for the executed file. […]

To take advantage of this wonderful opportunity, put

#! /bin/sh 

at the left margin of the first line of your shell scripts.

Blanks after ! are OK.

So spaces after the shebang have been around for quite a while, and indeed, Dennis Ritchie’s example is using them.

Note that early versions of Unix had a limit of 16 bytes in this interpreter line, so you couldn’t have many spaces there. Modern kernels usually have a higher limit¹.

^{¹ On Linux, it was initially 1023 bytes, then lowered to 127 bytes in 0.12 in 1992, then raised to 255 in 5.1 in 2019 (see commit); that shebang line will be truncated if longer though not if it's the path of the interpreter that would be truncated in which case the exec fails with ENOEXEC instead.}

Yes, blanks are allowed after the #!. There was even a (mistaken) thought that some systems might require it, but it has always just been optional.

For further reading try here

FYI, some versions of systemd fail to parse shebangs with a space. With a service like this:

[Service] Type=oneshot ExecStart=/root/foo.sh 

... and a shebang like this:

#! /bin/bash 

... you may get an error like this:

systemd[32834]: foo.service: Failed at step EXEC spawning /root/foo.sh: Exec format error 

Removing the space from the shebang fixes the error. So look out for that one.