How to symmetricize this nxn Identity matrix

documentclass{article}
usepackage{amsmath}
begin{document}
[ I=
begin{bmatrix}
1 & 0 & 0 & cdots & 0 
0 & 1 & 0 & cdots & 0 
0 & 0 & 1 & cdots & 0 
vdots& & & ddots &   
0 & 0 & 0 & cdots & 1
end{bmatrix}
]
end{document}

The problem is probably some package (or newcommand) you are using. You have too many dots in cdots, vdots and ddots, and because that the symmetry is lost. In the following code I have your matrix (I_1, with just 3 dots per command) and another matrix I_2. The latter is what I'd do.

documentclass{article}
usepackage   {amsmath}

begin{document}
[I_1=begin{bmatrix}
1      & 0      & 0      & cdots & 0      & 0 
0      & 1      & 0      & ddots &        & vdots 
vdots & ddots & ddots & ddots & 0      & 0 
0      & cdots & 0      & 1      & 0      & vdots 
vdots &        &        & ddots & ddots & 0 
0      & cdots & 0      & cdots & 0      & 1
end{bmatrix},quad I_2=begin{bmatrix}
1      & 0      & 0      & cdots & 0      & 0 
0      & 1      & 0      & cdots & 0      & 0 
0      & 0      & 1      & cdots & 0      & 0 
vdots & vdots & vdots & ddots & vdots & vdots
0      & 0      & 0      & cdots & 1      & 0 
0      & 0      & 0      & cdots & 0      & 1
end{bmatrix}.]
end{document}

It looks like this:

One more with nicematrix:

documentclass[12pt]{article}
usepackage{nicematrix}

begin{document}
    [
mathbf{I} =
begin{bNiceMatrix}
1   &       & Block{2-3}<huge>{0} 
    &   1   &        &      &       
    &       &   1    &      &       
Block{2-3}<huge>{0}
    &       &       & Ddots    &   
    &       &       &      &   1   
end{bNiceMatrix}
    ]
end{document}