How to add spacing between columns?

You can achieve spacing between columns using the col-md-offset-* classes, documented here. The spacing is consistent so that all of your columns line up correctly. To get even spacing and column size I would do the following:

<div class="row">
  <div class="col-md-5"></div>
  <div class="col-md-5 col-md-offset-2"></div>
</div>

In Bootstrap 4 use: offset-2 or offset-md-2

I was facing the same issue; and the following worked well for me.

<div class="row">
  <div class="col-md-6">
     <div class="col-md-12">
        Some Content.. 
     </div>
  </div>
  <div class="col-md-6">
     <div class="col-md-12">
        Some Second Content.. 
     </div>
  </div>
</div>

This will automatically render some space between the 2 divs.

I know I'm late to the party, but you could try spacing the boxes with padding.

<div class="col-md-6 box">
        <div class="inner">Hello</div>
</div>
<div class="col-md-6 box">
        <div class="inner">Hello</div>
</div>

CSS:

.box {
    padding: 0 5px 0 5px;
}
.box .inner {
    background-color: #fff;
}

Have a go at it